UNITED STATES

                       SECURITIES AND EXCHANGE COMMISSION

                             WASHINGTON, D.C. 20549

                               ------------------

                                    FORM 8-K

                                 CURRENT REPORT

                       PURSUANT TO SECTION 13 OR 15(d) OF

                       THE SECURITIES EXCHANGE ACT OF 1934

       Date of Report (Date of earliest event reported): January 25, 2005

                              MKS Instruments, Inc.
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             (Exact name of registrant as specified in its charter)

         Massachusetts                   0-23621                04-2277512
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(State or other jurisdiction of        (Commission             (IRS Employer
         incorporation)                File Number)          Identification No.)

90 Industrial Way, Wilmington, Massachusetts                             01887
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  (Address of principal executive offices)                            (Zip Code)

       Registrant's telephone number, including area code: (978) 284-4000

          (Former name or former address, if changed since last report)

Check the appropriate box below if the Form 8-K is intended to simultaneously
satisfy the filing obligation of the registrant under any of the following
provisions (see General Instruction A.2. below):

[ ]   Written communications pursuant to Rule 425 under the Securities Act (17
      CFR 230.425)

[ ]   Soliciting material pursuant to Rule 14a-12 under the Exchange Act (17 CFR
      240.14a-12)

[ ]   Pre-commencement communications pursuant to Rule 14d-2(b) under the
      Exchange Act (17 CFR 240.14d-2(b))

[ ]   Pre-commencement communications pursuant to Rule 13e-4(c) under the
      Exchange Act (17 CFR 240.13e4(c))

ITEM 1.01. ENTRY INTO A MATERIAL DEFINITIVE AGREEMENT.

      On January 25, 2005, the Compensation Committee of the Board of Directors
of MKS Instruments, Inc. ("MKS") approved the 2005 Management Incentive Program
(the "Program"). Pursuant to the terms of the Program, certain executive
officers of MKS are eligible to receive a cash bonus, calculated based on a
specified percentage of their respective 2005 base salary, upon achievement of
identified MKS' internal pro forma pre-tax earnings per share for the year
ending December 31, 2005.

      A copy of the Program is attached as Exhibit 10.1 to this Current Report
on Form 8-K.

ITEM 9.01 FINANCIAL STATEMENTS AND EXHIBITS.

(c) Exhibits

Exhibit Description - ------- ----------- 10.1 2005 Management Incentive Bonus Program of MKS Instruments, Inc.
SIGNATURE Pursuant to the requirements of the Securities Exchange Act of 1934, the Registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized. Date: January 27, 2005 MKS Instruments, Inc. By: /s/ Ronald C. Weigner ------------------------------------- Ronald C. Weigner, Vice President & Chief Financial Officer


                                                                    EXHIBIT 10.1

          Confidential Materials omitted and filed separately with the
        Securities and Exchange Commission. Asterisks denote omissions.

                            PERSONAL AND CONFIDENTIAL

                      2005 MKS MANAGEMENT INCENTIVE PROGRAM
                                    [FORM OF]
                PRO FORMA PRE-TAX EARNINGS PER SHARE (EPS) TABLE

                            (JANUARY 1 - DECEMBER 31)

YOUR TARGET BONUS UNDER THE 2005 MKS MANAGEMENT INCENTIVE PLAN IS ___%(1) OF
YOUR BASE EARNINGS. THE PAYOUT OF YOUR BONUS WILL BE ACHIEVED ACCORDING TO THE
SCHEDULE SHOWN IN THE CHART BELOW. FOR EXAMPLE, YOU WILL RECEIVE 50% OF YOUR
TARGET BONUS IF OUR PRO FORMA PRE-TAX EPS REACHES $**, AND 100% OF YOUR TARGET
BONUS IF OUR PRO FORMA PRE-TAX EPS REACHES $**. AT A PRO FORMA PRE-TAX EPS OF
$** OR MORE, YOU WOULD RECEIVE 200% OF YOUR TARGET BONUS, WHICH WOULD RESULT IN
THE PAYMENT OF A BONUS EQUAL TO ___%(2) OF YOUR 2005 BASE MKS EARNINGS.

Pro forma % of Target Pre-Tax EPS Bonus Earned - ----------- ------------ <=$** 0.0% $** 15.0% $** 25.0% $** 35.0% $** 50.0% $** 60.0% $** 75.0% $** 85.0% $** 100.0% $** 125.0% $** 150.0% $** 175.0% >=$** 200%
- -------- (1) Target bonus for John Bertucci is 75% of base earnings; for Leo Berlinghieri it is 60% of base earnings; for Jerry Colella and John A. Smith it is 50% of base earnings; for Robert Klimm, Ronald Weigner, William Stewart it is 40% of base earnings. (2) This number is double the number set forth in the paragraph above with respect to each of the executive officers. THIS INFORMATION IS EXTREMELY CONFIDENTIAL AND SHOULD BE TREATED AS SUCH. YOU SHOULD NOT DIVULGE THIS INFORMATION INSIDE OR OUTSIDE OF MKS INSTRUMENTS, INC.