As filed with the Securities and Exchange Commission on August 5, 2005 Registration No. 333-_____ ================================================================================ SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM S-8 REGISTRATION STATEMENT UNDER THE SECURITIES ACT OF 1933 MKS INSTRUMENTS, INC. (Exact Name of Registrant as Specified in Its Charter) MASSACHUSETTS 04-2277512 (State or Other Jurisdiction of (I.R.S. Employer Incorporation or Organization) Identification Number) 90 INDUSTRIAL WAY, WILMINGTON, MASSACHUSETTS 01887 (Address of Principal Executive Offices) (Zip Code) 2004 STOCK INCENTIVE PLAN (Full Title of the Plan) RONALD C. WEIGNER VICE PRESIDENT AND CHIEF FINANCIAL OFFICER MKS INSTRUMENTS, INC. 90 INDUSTRIAL WAY WILMINGTON, MA 01887 (Name and Address of Agent For Service) (978) 284-4000 (Telephone Number, Including Area Code, of Agent For Service) CALCULATION OF REGISTRATION FEE Proposed Maximum Proposed Maximum Title of Securities Amount to be Offering Price Per Aggregate Offering Amount of to be Registered Registered(1) Share Price Registration Fee - ------------------- ------------- ------------------ ------------------ ---------------- Common Stock, no 15,000,000 $18.83(2) $282,450,000(2) $33,245 par value per share (1) In accordance with Rule 416 under the Securities Act of 1933, as amended, this registration statement shall be deemed to cover any additional securities that may from time to time be offered or issued to prevent dilution resulting from stock splits, stock dividends or similar transactions. (2) Estimated solely for the purpose of calculating the registration fee pursuant to Rules 457(c) and 457(h) of the Securities Act of 1933, as amended, and based upon the average of the high and low prices of the registrant's Common Stock as reported on the Nasdaq National Market on August 2, 2005.

PART I INFORMATION REQUIRED IN THE SECTION 10(A) PROSPECTUS ITEM 1. PLAN INFORMATION. The information required by Item 1 is included in documents sent or given to participants in the plans covered by this registration statement pursuant to Rule 428(b)(1) of the Securities Act of 1933, as amended (the "Securities Act"). ITEM 2. REGISTRANT INFORMATION AND EMPLOYEE PLAN ANNUAL INFORMATION. The written statement required by Item 2 is included in documents sent or given to participants in the plans covered by this registration statement pursuant to Rule 428(b)(1) of the Securities Act. PART II INFORMATION REQUIRED IN THE REGISTRATION STATEMENT ITEM 3. INCORPORATION OF DOCUMENTS BY REFERENCE. The registrant is subject to the informational and reporting requirements of Sections 13(a), 14, and 15(d) of the Securities Exchange Act of 1934, as amended (the "Exchange Act"), and in accordance therewith files reports, proxy statements and other information with the Securities and Exchange Commission (the "Commission"). The following documents, which are on file with the Commission, are incorporated in this registration statement by reference: (a) The registrant's latest annual report filed pursuant to Section 13(a) or 15(d) of the Exchange Act or the latest prospectus filed pursuant to Rule 424(b) under the Securities Act that contains audited financial statements for the registrant's latest fiscal year for which such statements have been filed. (b) All other reports filed pursuant to Section 13(a) or 15(d) of the Exchange Act since the end of the fiscal year covered by the document referred to in (a) above. (c) The description of the securities contained in the registrant's registration statements on Form 8-A filed under the Exchange Act, including any amendment or report filed for the purpose of updating such description. All documents subsequently filed by the registrant pursuant to Sections 13(a), 13(c), 14 and 15(d) of the Exchange Act, prior to the filing of a post-effective amendment which indicates that all securities offered hereby have been sold or which deregisters all securities then remaining unsold, shall be deemed to be incorporated by reference in this registration statement and to be part hereof from the date of the filing of such documents. Any statement contained in a document incorporated or deemed to be incorporated by reference herein shall be deemed to be modified or superseded for the purposes of this registration statement to the extent that a statement contained herein or in any other subsequently filed document which also is or is deemed to be incorporated by reference herein modifies or supersedes such statement. Any statement so modified or superseded shall not be deemed, except as so modified or superseded, to constitute a part of this registration statement. ITEM 4. DESCRIPTION OF SECURITIES. Not applicable. - 2 -

ITEM 5. INTERESTS OF NAMED EXPERTS AND COUNSEL. Wilmer Cutler Pickering Hale and Dorr LLP has opined as to the legality of the securities being offered by this registration statement. ITEM 6. INDEMNIFICATION OF DIRECTORS AND OFFICERS. Section 8.51(a) of Chapter 156D of the Massachusetts General Laws provides that a corporation may indemnify its directors against expenses (including attorneys' fees), judgments, fines and amounts paid in settlement reasonably incurred in connection with any litigation or other legal proceeding brought against any director by virtue of his position as a director of the registrant unless he is deemed to have not acted in good faith in the reasonable belief that his action was in the best interest of the corporation. Section 8.52 of Chapter 156D provides that a corporation must indemnify a director who is wholly successful, on the merits or otherwise, in the defense of any proceeding to which the director was a party because he was a director of the corporation against reasonable expenses incurred by him in connection with the proceeding. Section 8.56(a) of Chapter 156D provides that a corporation may indemnify its officers to the same extent as its directors and, for officers that are not directors, to the extent provided by (i) the articles of organization, (ii) the bylaws, (iii) a vote of the board of directors or (iv) a contract. In all instances, the extent to which a corporation provides indemnification to its officers under Section 8.56(a) is optional. Section 8.56(c) provides that an officer of a corporation who is not a director is entitled to mandatory indemnification under Section 8.52. In its Restated Articles of Organization (the "Articles of Organization"), the registrant has elected to commit to provide indemnification to its directors and officers in specified circumstances. Generally, Article 6 of the registrant's Articles of Organization provides that the registrant shall indemnify directors and officers of the registrant against liabilities and expenses arising out of legal proceedings brought against them by reason of their status as directors or officers or by reason of their agreeing to serve, at the request of the registrant, as a director or officer with another organization. Under this provision, a director or officer of the registrant shall be indemnified by the registrant for all expenses (including attorneys' fees), judgments and fines, and amounts paid in settlement of such proceedings, even if he is not successful on the merits, unless finally adjudicated not to have acted in good faith in the reasonable belief that his action was in the best interests of the registrant. The registrant shall not indemnify an officer or director who initiates the legal proceeding, unless the initiation was approved by the Board of Directors. The Board of Directors may authorize advancing litigation expenses to a director or officer at his request upon receipt of an undertaking by any such director or officer to repay such expenses if it is ultimately determined that he is not entitled to indemnification for such expenses. Section 2.02(b)(4) of Chapter 156D of the Massachusetts General Laws allows a corporation to eliminate or limit the personal liability of a director of a corporation to the corporation for monetary damages for a breach of fiduciary duty as a director notwithstanding any provision of law imposing such liability, except where the director breached his duty of loyalty to the corporation or its shareholders, failed to act in good faith, engaged in intentional misconduct or knowingly violated a law, authorized the payment of an improper distribution or obtained an improper personal benefit. Article 6A of the registrant's Articles of Organization eliminates the personal liability of the registrant's directors to the registrant or its stockholders for monetary damages for breach of a director's fiduciary duty, except to the extent Chapter 156D of the Massachusetts General Laws prohibits the elimination or limitation of such liability. - 3 -

The registrant has obtained directors and officers' liability insurance for the benefit of its directors and certain of its officers. ITEM 7. EXEMPTION FROM REGISTRATION CLAIMED. Not applicable. ITEM 8. EXHIBITS. The Exhibit Index immediately preceding the exhibits is incorporated herein by reference. ITEM 9. UNDERTAKINGS. 1. Item 512(a) of Regulation S-K. The undersigned registrant hereby undertakes: (1) To file, during any period in which offers or sales are being made, a post-effective amendment to this registration statement: (i) To include any prospectus required by Section 10(a)(3) of the Securities Act; (ii) To reflect in the prospectus any facts or events arising after the effective date of the registration statement (or the most recent post-effective amendment thereof) which, individually or in the aggregate, represent a fundamental change in the information set forth in the registration statement; and (iii) To include any material information with respect to the plan of distribution not previously disclosed in the registration statement or any material change to such information in the registration statement; provided, however, that paragraphs (i) and (ii) do not apply if the information required to be included in a post-effective amendment by those paragraphs is contained in periodic reports filed with or furnished to the Commission by the registrant pursuant to Section 13 or Section 15(d) of the Exchange Act that are incorporated by reference in the registration statement. (2) That, for the purpose of determining any liability under the Securities Act, each such post-effective amendment shall be deemed to be a new registration statement relating to the securities offered therein, and the offering of such securities at that time shall be deemed to be the initial bona fide offering thereof. (3) To remove from registration by means of a post-effective amendment any of the securities being registered which remain unsold at the termination of the offering. 2. Item 512(b) of Regulation S-K. The undersigned registrant hereby undertakes that, for purposes of determining any liability under the Securities Act, each filing of the registrant's annual report pursuant to Section 13(a) or Section 15(d) of the Exchange Act that is incorporated by reference in the registration statement shall be deemed to be a new registration statement relating to the securities offered therein, and the offering of such securities at that time shall be deemed to be the initial bona fide offering thereof. - 4 -

3. Item 512(h) of Regulation S-K. Insofar as indemnification for liabilities arising under the Securities Act may be permitted to directors, officers and controlling persons of the registrant pursuant to the foregoing provisions, or otherwise, the registrant has been advised that in the opinion of the Securities and Exchange Commission such indemnification is against public policy as expressed in the Securities Act and is, therefore, unenforceable. In the event that a claim for indemnification against such liabilities (other than the payment by the registrant of expenses incurred or paid by a director, officer or controlling person of the registrant in the successful defense of any action, suit or proceeding) is asserted by such director, officer or controlling person in connection with the securities being registered, the registrant will, unless in the opinion of its counsel the matter has been settled by controlling precedent, submit to a court of appropriate jurisdiction the question whether such indemnification by it is against public policy as expressed in the Securities Act and will be governed by the final adjudication of such issue. - 5 -

SIGNATURES Pursuant to the requirements of the Securities Act, the Registrant certifies that it has reasonable grounds to believe that it meets all of the requirements for filing on Form S-8 and has duly caused this Registration Statement to be signed on its behalf by the undersigned, thereunto duly authorized, in Wilmington, Massachusetts on August 1, 2005. MKS INSTRUMENTS, INC. By: /s/ Leo Berlinghieri ------------------------ Leo Berlinghieri President, Chief Executive Officer and Director POWER OF ATTORNEY AND SIGNATURES We, the undersigned officers and directors of MKS Instruments, Inc. hereby severally constitute and appoint Leo Berlinghieri, Ronald C. Weigner and Kathleen F. Burke, and each of them singly, our true and lawful attorneys with full power to them, and each of them singly, to sign for us and in our names in the capacities indicated below, the Registration Statement on Form S-8 filed herewith and any and all subsequent amendments to said Registration Statement, and generally to do all such things in our names and behalf in our capacities as officers and directors to enable MKS Instruments, Inc. to comply with all requirements of the Securities and Exchange Commission, hereby ratifying and confirming our signatures as they may be signed by said attorneys, or any of them, to said Registration Statement and any and all amendments thereto. Pursuant to the requirements of the Securities Act, this Registration Statement has been signed by the following persons in the capacities and on the dates indicated. SIGNATURE TITLE DATE --------- ----- ---- President, Chief Executive August 1, 2005 /s/ Leo Berlinghieri Officer and Director (Principal - ---------------------- Executive Officer) Leo Berlinghieri /s/ Ronald C. Weigner Vice President and Chief August 1, 2005 - ---------------------- Financial Officer (Principal Ronald C. Weigner Financial Officer and Principal Accounting Officer) /s/ John R. Bertucci Director August 1, 2005 - ---------------------- John R. Bertucci /s/ Robert R. Anderson Director August 1, 2005 - ---------------------- Robert R. Anderson /s/ James G. Berges Director August 1, 2005 - ---------------------- James G. Berges /s/ Richard S. Chute Director August 1, 2005 - ---------------------- Richard S. Chute /s/ Hans-Jochen Kahl Director August 1, 2005 - ---------------------- Hans-Jochen Kahl /s/ Owen W. Robbins Director August 1, 2005 - ---------------------- Owen W. Robbins /s/ Louis P. Valente Director August 1, 2005 - ---------------------- Louis P. Valente - 6 -

EXHIBIT INDEX Exhibit Number Description ------- ----------- 4.1 (1) Restated Articles of Organization of the Registrant. 4.2 (2) Articles of Amendment. 4.3 (3) Articles of Amendment. 4.4 (4) Amended and Restated By-Laws of the Registrant. 4.5 (4) Specimen Certificate for Common Stock of the Registrant. 5.1 Opinion of Wilmer Cutler Pickering Hale and Dorr LLP. 23.1 Consent of Wilmer Cutler Pickering Hale and Dorr LLP (included in Exhibit 5.1). 23.2 Consent of PricewaterhouseCoopers LLP. 24.1 Power of Attorney (included in the signature pages of this Registration Statement). - ---------------- (1) Incorporated herein by reference from the Registrant's Registration Statement on Form S-4 (File No. 333-49738) filed with the Securities and Exchange Commission on November 13, 2000. (2) Incorporated herein by reference from the Registrant's Quarterly Report on Form 10-Q (File No. 000-23621) for the quarter ended June 30, 2001. (3) Incorporated herein by reference from the Registrant's Quarterly Report on Form 10-Q (File No. 000-23621) for the quarter ended June 30, 2002. (4) Incorporated herein by reference from the Registrant's Registration Statement on Form S-1 (File No. 333-71363) filed with the Securities and Exchange Commission on January 28, 1999, as amended. - 7 -

Exhibit 5.1 [Wilmer Cutler Pickering Hale and Dorr LLP Letterhead] August 4, 2005 MKS Instruments, Inc. 90 Industrial Way Wilmington, MA 01887 Re: 2004 Stock Incentive Plan Ladies and Gentlemen: We have assisted in the preparation of a Registration Statement on Form S-8 (the "Registration Statement") to be filed with the Securities and Exchange Commission (the "Commission") under the Securities Act of 1933, as amended (the "Securities Act"), relating to an aggregate of 15,000,000 shares of common stock, no par value per share (the "Shares"), of MKS Instruments, Inc., a Massachusetts corporation (the "Company"), issuable under the Company's 2004 Stock Incentive Plan (the "Plan"). We have examined the Articles of Organization and By-Laws of the Company, each as amended and restated to date, and originals, or copies certified to our satisfaction, of all pertinent records of the meetings of the directors and stockholders of the Company, the Registration Statement and such other documents relating to the Company as we have deemed material for the purposes of this opinion. In our examination of the foregoing documents, we have assumed the genuineness of all signatures, the authenticity of all documents submitted to us as originals, the conformity to original documents of all documents submitted to us as certified, photostatic or other copies, the authenticity of the originals of any such documents and the legal competence of all signatories to such documents. We assume that the appropriate action will be taken, prior to the offer and sale of the shares in accordance with the Plan, to register and qualify the shares for sale under all applicable state securities or "blue sky" laws. We express no opinion herein as to the laws of any state or jurisdiction other than the state laws of The Commonwealth of Massachusetts and the federal laws of the United States of America. It is understood that this opinion is to be used only in connection with the offer and sale of the Shares while the Registration Statement is in effect. Please note that we are opining only as to the matters expressly set forth herein, and no opinion should be inferred as to any other matters.

MKS Instruments, Inc. August 4, 2005 Page 2 Based on the foregoing, we are of the opinion that the Shares have been duly authorized for issuance and, when the Shares are issued and paid for in accordance with the terms and conditions of the Plan, the Shares will be validly issued, fully paid and nonassessable. We hereby consent to the filing of this opinion with the Securities and Exchange Commission in connection with the Registration Statement in accordance with the requirements of Item 601(b)(5) of Regulation S-K under the Securities Act. In giving such consent, we do not hereby admit that we are in the category of persons whose consent is required under Section 7 of the Securities Act or the rules and regulations of the Commission. Very truly yours, WILMER CUTLER PICKERING HALE AND DORR LLP By: /s/ Mark G. Borden ---------------------------- Mark G. Borden, a Partner

CONSENT OF INDEPENDENT REGISTERED PUBLIC ACCOUNTING FIRM We hereby consent to the incorporation by reference in this Registration Statement on Form S-8 of our report dated March 14, 2005 relating to the financial statements, financial statement schedule, management's assessment of the effectiveness of internal control over financial reporting and the effectiveness of internal control over financial reporting, of MKS Instruments, Inc., which appears in MKS Instruments, Inc.'s Annual Report on Form 10-K for the year ended December 31, 2004. /s/ PricewaterhouseCoopers LLP Boston, Massachusetts August 2, 2005