Exhibit 23.1

Consent of Independent Registered Public Accounting Firm

We consent to the use of our report dated April 8, 2022, with respect to the consolidated financial statements of Atotech Limited, incorporated herein by reference.

/s/ KPMG AG Wirtschaftsprüfungsgesellschaft

Berlin, Germany

August 24, 2022

Exhibit 99.3

UNAUDITED PRO FORMA CONDENSED COMBINED FINANCIAL INFORMATION

The unaudited pro forma condensed combined financial information (“Unaudited Pro Forma Financial Information”) included herein presents the unaudited pro forma condensed combined balance sheet (“Unaudited Pro Forma Balance Sheet”) and the unaudited pro forma condensed combined statements of operations (“Unaudited Pro Forma Statements of Operations”) based upon the historical financial statements of MKS Instruments, Inc. (“MKS” or the “Company”) and Atotech Limited (“Atotech”), after giving effect to the Company’s acquisition of Atotech (the “Atotech Acquisition”) and the Atotech Financing Transaction (as defined below) (collectively, the “Transaction”), and the adjustments described in the accompanying notes.

The Unaudited Pro Forma Statements of Operations for the six months ended June 30, 2022 and the year ended December 31, 2021 give effect to the Transaction as if it had occurred on January 1, 2021. The Unaudited Pro Forma Balance Sheet as of June 30, 2022 gives effect to the Transaction as if it had occurred on June 30, 2022.

The Unaudited Pro Forma Financial Information set out below has been prepared in accordance with Article 11 of Regulation S-X, as amended by the U.S. Securities and Exchange Commission (“SEC”) Final Rule Release No. 33 10786, Amendments to Financial Disclosures About Acquired and Disposed Businesses (“Regulation S-X”), using accounting policies in accordance with principles generally accepted in the United States of America (“U.S. GAAP”).

The Unaudited Pro Forma Financial Information reflects transaction accounting adjustments management believes are necessary to present fairly the Unaudited Pro Forma Balance Sheet and Unaudited Pro Forma Statements of Operations. The financial information presented may differ from actual amounts due to rounding for presentation purposes.

The Unaudited Pro Forma Financial Information has been prepared for illustrative purposes only. The hypothetical financial position or results included in the Unaudited Pro Forma Financial Information may differ from the Company’s actual financial position or results following the Transaction. The Unaudited Pro Forma Financial Information has been prepared on the basis set out in the notes below and has been prepared in a manner consistent with the accounting policies applied by the Company in its historical financial statements for the six months ended June 30, 2022 and the year ended December 31, 2021. In preparing the Unaudited Pro Forma Financial Information, no adjustments have been made to reflect the potential operating synergies, dis-synergies, and administrative cost savings or the costs of integration activities that could result from the combination of MKS and Atotech.

Atotech Acquisition

On August 17, 2022 (the “Effective Date”), the Company completed its previously announced Atotech Acquisition, through the acquisition of the entire issued and to be issued share capital of Atotech by Atotech Manufacturing, Inc. (“Bidco”), a Delaware corporation and indirect wholly owned subsidiary of the Company. The Atotech Acquisition was implemented by means of a scheme of arrangement under the laws of Jersey (the “Scheme”) pursuant to the definitive agreement entered into by the Company and Atotech on July 1, 2021, as amended by the Letter Agreement dated October 29, 2021 by and among the Company, Atotech and Bidco and as further amended by the Amendment to the Implementation Agreement dated April 1, 2022 by and among the Company, Atotech and Bidco (together, the “Implementation Agreement”). On the Effective Date, pursuant to the Scheme and in accordance with terms and conditions of the Implementation Agreement, Bidco acquired each issued and outstanding ordinary share of Atotech in exchange for per share consideration of $16.20 in cash and 0.0552 of a share of Company common stock. Atotech ordinary shares have been de-listed from The New York Stock Exchange effective as of the close of trading on August 16, 2022.

The Company funded the payment of the aggregate consideration with a combination of cash on hand and the proceeds from the New Term Loan Facility (as defined below). As a result of the Atotech Acquisition, the Company issued an aggregate of 10,748,728 shares of Company common stock to the former Atotech shareholders.

The Atotech Acquisition was accounted for as a business combination using the acquisition method with MKS as the accounting acquirer in accordance with ASC 805. In concluding that MKS is the accounting acquirer, the Company considered the structure of the transaction, relative outstanding share ownership, the composition of the combined company’s board of directors, the relative size of MKS and Atotech, and the designation of certain senior management positions of the combined company.

Atotech Financing Transaction

On the Effective Date, in connection with the completion of the Atotech Acquisition, the Company entered into a new credit agreement with JPMorgan Chase Bank, N.A., as administrative agent and collateral agent, Barclays Bank PLC, and the lenders from time to time party thereto (the “New Credit Agreement”), which provides for (i) a senior secured term loan facility (the “New Term Loan Facility”) comprised of three tranches: a $1,000,000,000 loan (the “Term Loan A”), a $3,600,000,000 loan (the “USD Term Loan B”) and a €600,000,000 loan (the “EUR Term Loan B” and, together with the USD Term Loan B, the “Term Loan B”), each of which were borrowed in full on the Effective Date, and (ii) a senior secured revolving credit facility of $500,000,000 (the “New Revolving Facility” and, together with the New Term Loan Facility, the “New Credit Facilities”).

Borrowings under the New Credit Facilities bear interest at a rate per annum equal to, at the Company’s option, any of the following, plus, in each case, an applicable margin: (a) with respect to the Term Loan A, the USD Term Loan B and the New Revolving Facility, (x) a base rate determined by reference to the highest of (1) the federal funds effective rate plus 0.50%, (2) the prime rate quoted in The Wall Street Journal, or (3) a Term SOFR rate (plus an applicable credit spread adjustment) for an interest period of one month, plus 1.00%; and (y) a Term SOFR rate (plus an applicable credit spread adjustment) for the interest period relevant to such borrowing, subject, solely with respect to the USD Term Loan B, to a rate floor of 0.50%; and (b) with respect to the EUR Term Loan B, a EURIBOR rate determined by reference to the costs of funds for Euro deposits for the interest period relevant to such borrowing adjusted for certain additional costs, subject to a EURIBOR rate floor of 0.0%. The Term Loan A was issued with original issue discount of 0.25% of the principal amount thereof. The USD Term Loan B and the EUR Term Loan B were issued with original issue discount of 2.00% of the principal amount thereof. The applicable margin for borrowings under the Term Loan A is 1.50% with respect to base rate borrowings and 2.50% with respect to Term SOFR borrowings. The applicable margin for borrowings under the USD Term Loan B is 1.75% with respect to base rate borrowings and 2.75% with respect to Term SOFR borrowings. The applicable margin for borrowings under the EUR Term Loan B is 3.00%. The initial applicable margin for borrowings under the New Revolving Facility is 1.50% with respect to base rate borrowings and 2.50% with respect to Term SOFR borrowings. Commencing with the delivery of financial statements with respect to the first quarter ending after the closing of the New Credit Agreement, the applicable margin for borrowings under the New Revolving Facility is subject to adjustment each fiscal quarter, based on the Company’s first lien net leverage ratio during the preceding quarter.

In addition to paying interest on outstanding principal under the New Credit Facilities, the Company is required to pay a commitment fee in respect of the unutilized commitments under the New Revolving Facility. The initial commitment fee is 0.375% per annum. Commencing with the delivery of financial statements with respect to the first quarter ending after the closing of the New Credit Agreement, the commitment fee is subject to downward adjustment based on the Company’s first lien net leverage ratio during the preceding quarter. The Company must also pay customary letter of credit fees and agency fees.

Under the New Credit Agreement, the Company is required to prepay outstanding term loans, subject to certain exceptions, with portions of its annual excess cash flow as well as with the net cash proceeds of certain asset sales, certain casualty and condemnation events and the incurrence or issuance of certain debt. The Company is also required to make scheduled quarterly payments each equal to 1.25% of the original principal amount of the Term Loan A (increasing to 1.875% in years 3 and 4 and 2.50% in year 5) and 0.25% of the original principal amount of the USD Term Loan B and the EUR Term Loan B, beginning with the fiscal quarter ending December 31, 2022, with the balance due thereunder on the fifth anniversary of the closing date in the case of the Term Loan A and the seventh anniversary of the closing date in the case of the USD Term Loan B and the EUR Term Loan B.

All obligations under the New Credit Facilities are guaranteed by certain of the Company’s domestic subsidiaries, and are secured by substantially all of the Company’s assets and the assets such subsidiary guarantors, in each case subject to certain exceptions and exclusions.

The New Credit Agreement contains customary representations and warranties, affirmative and negative covenants and provisions relating to events of default. If an event of default occurs, the lenders under the New Credit Facilities will be entitled to take various actions, including the acceleration of amounts due under the New Credit Facilities and all actions permitted to be taken by a secured creditor.

Under the Term Loan A and the New Revolving Facility, so long as any Term Loan A loans (or commitments in respect thereof) are outstanding as of the end of any fiscal quarter, the Company may not allow its total net leverage ratio as of the end of such fiscal quarter to be greater than 5.50 to 1.00, with an annual step-down of 0.25:1.00 and subject to a step-up of 0.50:1.00 for the four full fiscal quarter period following any material acquisition, not to exceed 5.50 to 1.00. In addition, as of the end of any fiscal quarter of the Company when the aggregate amount of loans outstanding under the New Revolving Facility (net of (a) all letters of credit (whether cash collateralized or not) and (b) unrestricted cash of the Company and its restricted subsidiaries) exceeds 35% of the aggregate amount of all commitments under the New Revolving Facility in effect as of such date, the Company may not allow its first lien net leverage ratio as of the end of each such fiscal quarter to be greater than 6.00 to 1.00. The USD Term Loan B and the EUR Term Loan B are not subject to any financial maintenance covenant.

The Company used the proceeds of the New Term Loan Facility and cash on hand on the Effective Date, to (i) pay the cash portion of the purchase price of the Atotech Acquisition, (ii) refinance certain existing indebtedness of Atotech, (iii) refinance the Company’s existing senior secured term loan credit facility and existing senior secured revolving credit facility and (iv) pay fees and expenses related to the foregoing.

On the Effective Date, in connection with the entry into the New Credit Agreement described above, the Company terminated and prepaid its existing senior secured term loan credit facility and senior secured revolving credit facility.

The New Credit Facilities are referred to as the “Atotech Financing Transaction.”

Unaudited Pro Forma Condensed Combined Balance Sheet as of June 30, 2022

See accompanying notes to the Unaudited Pro Forma Condensed Combined Financial Information, which are an integral part of these statements.

Unaudited Pro Forma Condensed Combined Statement of Operations for the six months ended June 30, 2022

See accompanying notes to the Unaudited Pro Forma Condensed Combined Financial Information, which are an integral part of these statements.

Unaudited Pro Forma Condensed Combined Statement of Operations for the year ended December 31, 2021

See accompanying notes to the Unaudited Pro Forma Condensed Combined Financial Information, which are an integral part of these statements.

Notes to the Unaudited Pro Forma Condensed Combined Financial Information

Note 1 — Description of Transaction and Basis of Presentation

The Unaudited Pro Forma Financial Information has been prepared based on U.S. GAAP and pursuant to Regulation S-X and presents the Unaudited Pro Forma Balance Sheet and Unaudited Pro Forma Statements of Operations of the combined company based upon the historical financial information of MKS and Atotech, after giving effect to the Transaction. Unless otherwise indicated, amounts are shown in millions of U.S. dollars rounded to the millions in accordance with standard commercial practice, which may result in rounding differences within the tables of the Unaudited Pro Forma Financial Information.

The Unaudited Pro Forma Financial Information is not necessarily indicative of what MKS’ consolidated statements of operations or consolidated balance sheet would have been had the Transaction been completed as of the date indicated or will be for any future periods. The Unaudited Pro Forma Financial Information does not purport to project the future financial position or results of operations of MKS following the Transaction. The Unaudited Pro Forma Financial Information reflects accounting adjustments related to the Transaction management believes are necessary to present fairly the MKS Unaudited Pro Forma Balance Sheet and Unaudited Pro Forma Statements of Operations assuming the Transaction had been consummated as of June 30, 2022 and January 1, 2021, respectively. The accounting related Transaction adjustments are based on currently available information and assumptions management believes are, under the circumstances and given the information available at this time, reasonable, and reflective of adjustments necessary to report MKS financial condition and results of operations as a result of the closing of the Transaction.

MKS has concluded that the Atotech Acquisition represents a business combination pursuant to ASC 805. As of the date of this filing, the calculations necessary to estimate the fair values of the assets acquired and liabilities assumed have been performed based on publicly available benchmarking information as well as a variety of other assumptions, including market participant assumptions for the Atotech Acquisition. The Company will continue to refine its identification and valuation of assets acquired and the liabilities assumed as further information becomes available. Using the total consideration for the Atotech Acquisition, MKS has preliminarily allocated the purchase price to such assets and liabilities as of June 30, 2022.

The preliminary purchase price allocation has been used to prepare pro forma adjustments in the Unaudited Pro Forma Financial Information. The pro forma adjustments are based on preliminary estimates and currently available information and assumptions that MKS management believes are reasonable. The final purchase price allocation could differ materially from the preliminary purchase price allocation. The final purchase price allocation may include changes in the allocation to intangible assets and goodwill based on the results of certain valuations and other studies that have yet to be completed, and other changes to assets and liabilities. The accompanying notes to the Unaudited Pro Forma Financial Information describe how such adjustments were derived and presented in the Unaudited Pro Forma Balance Sheet and Unaudited Pro Forma Statements of Operations. Changes in facts and circumstances or discovery of new information may result in revised estimates. As a result, there may be material adjustments to the Unaudited Pro Forma Financial Information.

The Unaudited Pro Forma Financial Information has been compiled in a manner consistent with the accounting policies adopted by MKS and reflects certain adjustments to the historical financial information of Atotech to align to the accounting policies of MKS based on a preliminary review of the accounting policies of Atotech. Certain historical financial statement caption amounts for Atotech have been reclassified or combined to align to MKS presentation and disclosure requirements.

The Unaudited Pro Forma Financial Information has been developed from, and should be read in conjunction with, the audited consolidated financial statements of MKS and accompanying notes thereto included in MKS’ Annual Report filed on Form 10-K as of and for the year ended December 31, 2021, the unaudited condensed consolidated financial statements of MKS and accompanying notes thereto included in MKS’ Quarterly Report filed on Form 10-Q as of and for the six months ended June 30, 2022, as well as, the audited consolidated financial statements of Atotech and accompanying notes thereto and the unaudited interim condensed consolidated financial statements of Atotech and accompanying notes thereto included in Exhibits 99.1 and 99.2, respectively, of this MKS’ Current Report on Form 8-K/A filed with the Securities and Exchange Commission on August 24, 2022 (the “Form 8-K/A”), of which this Exhibit 99.3 is a part.

Note 2 —   MKS historical financial statements

The results of MKS for the year ended December 31, 2021 have been extracted from the audited consolidated financial statements of MKS, as set out in MKS’ Annual Report on Form 10-K for the year ended December 31, 2021, filed with the SEC on February 28, 2022. The results of MKS as of and for the six months ended June 30, 2022 have been extracted from the unaudited condensed consolidated financial statements of MKS, as set out in MKS’ Quarterly Report on Form 10-Q as of and for the six months ended June 30, 2022, filed with the SEC on August 3, 2022.

Note 3 —   Atotech reclassifications and IFRS to U.S. GAAP adjustments

The historical financial statements of Atotech were prepared in accordance with International Financial Reporting Standards as issued by the International Accounting Standards Board (“IFRS”), which differ in certain significant respects from U.S. GAAP as applied by MKS. Adjustments for material differences were made to the financial statements of Atotech to convert them from IFRS to U.S. GAAP and to align to accounting policies of MKS after evaluating potential areas of differences.

Refer below for impacted line items and adjustments to the Unaudited Pro Forma Balance Sheet as of June 30, 2022:

(a) The net assets of Atotech as of June 30, 2022 have been extracted from the Unaudited Interim Consolidated Statement of Financial Position as of June 30, 2022, as set out in Atotech’s Form 6-K filed with the SEC on August 9, 2022.

Certain items presented by Atotech under IFRS have been reclassified to align to the presentation used by MKS under U.S. GAAP. The following reclassifications were made to the Unaudited Pro Forma Balance Sheet presentation:

(b) Reclassification of non-current other financial assets and non-current other non-financial assets to other assets.

(c) Reclassification of tax assets, other financial assets and other non-financial assets to other current assets.

(d) Reclassification of paid-in surplus and retained earnings and currency translation adjustment and other reserves to additional paid-in-capital and accumulated other comprehensive income, respectively.

(e) Reclassification of provisions to other non-current liabilities.

(f) Reclassification of tax liabilities related to sales tax payable to other current liabilities.

(g) Reclassification of other financial liabilities to other current liabilities.

(h) Reclassification of provisions to tax liabilities and other current liabilities.

(i) Reclassification of other non-financial liabilities to deferred revenue and customer advances and other current liabilities.

(j) Reclassification of accrued compensation from other current liabilities.

Certain items presented by Atotech under IFRS have been adjusted to align with the accounting of MKS under U.S. GAAP. The following IFRS to U.S. GAAP adjustments were made to the Unaudited Pro Forma Balance Sheet:

(k) Reflects an adjustment to reverse capitalized research and development (“R&D”) expenses. Under IFRS, certain development expenses can be capitalized and subsequently amortized once projects are complete and ready for use. In accordance with IAS 12, costs related to capitalized R&D are recognized as an intangible asset while also creating a temporary tax difference of carrying amount versus a tax basis of zero. Under U.S. GAAP, R&D costs are not capitalizable but rather are expensed as incurred. As such, the relevant deferred tax liability is also reversed. Further, the related amortization of capitalized R&D has been reversed in the Unaudited Pro Forma Statements of Operations.

Refer below for impacted line items and adjustments to the Unaudited Pro Forma Statement of Operations for the six months ended June 30, 2022:

Refer below for impacted line items and adjustments to the Unaudited Pro Forma Statement of Operations for the year ended December 31, 2021:

(a) The results of Atotech for the six months ended June 30, 2022 have been extracted from the Unaudited Interim Consolidated Statement of Comprehensive Income/(Loss), as set out in Atotech’s Form 6-K filed on August 9 2022. The results of Atotech for the year ended December 31, 2021 have been extracted from the Audited Consolidated Statement of Comprehensive Income, as set out in Atotech’s Form 20-F filed with the SEC on April 11, 2022.

Certain items presented by Atotech under IFRS have been reclassified to align to the presentation used by MKS under U.S. GAAP. The following reclassifications were made to the Unaudited Pro Forma Statements of Operations presentation:

(b) Reclassification of depreciation expense to align to the accounting policies of MKS to report depreciation within the respective financial statement line items.

(c) Reclassification of historical amortization of intangible assets to align to MKS financial presentation of intangible asset amortization expense.

(d) Reclassification of certain historical logistics costs to align to MKS’ financial presentation.

(e) Reclassification to separately state interest income to align to MKS’ financial presentation.

Certain items presented by Atotech under IFRS have been adjusted to align to the accounting policies of MKS under U.S. GAAP. The following IFRS to U.S. GAAP adjustments were made to the Unaudited Pro Forma Statements of Operations:

(f) Reflects adjustment to record additional R&D expense that was capitalized by Atotech under IFRS. Under IFRS, certain development expenses can be capitalized, and subsequently amortized once projects are complete and ready for use. Under U.S. GAAP, development expenses are not capitalized. Additionally, reflects the reversal of the related amortization expense and the tax impact.

(g) Reflects reclassification of selling, general, and administrative expense from depreciation expense related to leased assets. Under IFRS, lessee-leases are not classified as operating or finance leases. Rather, a single recognition and measurement model is applied to all leases, which results in nearly all leases under IFRS being treated similarly to finance leases under U.S. GAAP. Under U.S. GAAP, leases are classified as either operating or finance leases on the basis of specific lease classification criteria. Management performed a preliminary assessment and concluded that the majority of the Atotech leases would be classified as operating leases under U.S. GAAP with lease expense recognized on a straight-line basis as part of cost of products and selling, general, and administrative expenses.

(h) Reflects the reversal of other expense (income), related tax impact, and recognition of additional interest expense to adjust amounts recognized related to financial instruments that were bifurcated under IFRS, which includes embedded derivatives of prepayment options in financing notes that Atotech held, as these prepayment options would not be bifurcated under U.S. GAAP.

Note 4 — Atotech Acquisition adjustments

4(a) Preliminary purchase price and allocation:

As a result of the Atotech Acquisition, MKS acquired all 195 million issued and outstanding common shares of Atotech and upon the closing, Atotech stockholders received consideration of $16.20 in cash and 0.0552 shares of MKS common stock for each common share of Atotech. MKS funded the cash portion of the Atotech Acquisition with a combination of available cash on hand and proceeds from the Atotech Financing Transaction. The remaining proceeds from the Atotech Financing Transaction on the Effective Date were used to refinance certain existing indebtedness of Atotech, to refinance MKS’ existing senior secured term loan credit facility and existing senior secured revolving credit facility and to pay fees and expenses related to the Atotech Acquisition and Atotech Financing Transaction.

MKS performed a preliminary analysis of the fair value of Atotech assets and liabilities based on publicly available benchmarking information, as well as a variety of other assumptions, including market participant assumptions. The following table summarizes the allocation of the preliminary purchase price as of the acquisition date:

4(b) Represents the estimated fair value of intangible assets acquired. This includes the total acquired finite-lived intangible assets less the historical intangible assets recorded by Atotech.