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 7, 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          (Commission               (IRS Employer
      of 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


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          (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 8.01.  OTHER EVENTS.

     On January 11, 2005, MKS Instruments, Inc. ("MKS") issued a press release
announcing that on January 7, 2005, it had accelerated the vesting of certain
unvested options awarded to its employees and officers under its stock option
plans. The acceleration applies only to those options with an exercise price of
$23.00 or greater. The closing sale price of MKS stock on the Nasdaq National
Market on January 7, 2005 was $16.03.

     Under the recently issued Financial Accounting Standards Board Statement
No. 123R, "Share-Based Payment" ("FAS 123R"), MKS will be required to apply the
expense recognition provisions under FAS 123R beginning July 1, 2005. The
Company believes that accelerating the vesting of the identified stock options
will reduce the Company's compensation charge in periods subsequent to June 30,
2005.

     The full text of the press release is attached as Exhibit 99.1 to this
Current Report on Form 8-K.

ITEM 9.01   FINANCIAL STATEMENTS AND EXHIBITS

(c) Exhibits

     Exhibit
     Number    Description
     ------    -----------

     99.1      Press release dated January 11, 2005 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 11, 2005                          MKS Instruments, Inc.



                                                By: /s/ Ronald C. Weigner
                                                    ----------------------------
                                                    Ronald C. Weigner,
                                                    Vice President &  
                                                    Chief Financial Officer