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): February 10, 2005

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

        Massachusetts                          0-23621                  04-2277512
(State or other jurisdiction of              (Commission               (IRS Employer
      incorporation)                         File Number)            Identification No.)
90 Industrial Way, Wilmington, Massachusetts                                     01887
 (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

[ ]   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))


      On February 10, 2005, MKS Instruments, Inc. announced its financial
results for the quarter and year ended December 31, 2004. The full text of the
press release is attached as Exhibit 99.1 to this Current Report on Form 8-K.

         The information in this Form 8-K and the Exhibit attached hereto shall
not be deemed "filed" for purposes of Section 18 of the Securities Exchange Act
of 1934 (the "Exchange Act") or otherwise subject to the liabilities of that
section, nor shall it be deemed incorporated by reference in any filing under
the Securities Act of 1933 or the Exchange Act, except as expressly set forth by
specific reference in such a filing.


      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:  February 10, 2005                 MKS Instruments, Inc.

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

                                  EXHIBIT INDEX

Exhibit No.                     Description
-----------                     -----------

99.1                            Press release dated February 10, 2005