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): October 19, 2004

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

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


     90 Industrial Way, Wilmington,                       01887
             Massachusetts
(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 2.02.  RESULTS OF OPERATIONS AND FINANCIAL CONDITION.

      On October 19, 2004, MKS Instruments, Inc. announced its financial results
for the quarter ended September 30, 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.

                                    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:  October 19, 2004            MKS Instruments, Inc.

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

                                  EXHIBIT INDEX



Exhibit No.                     Description
-----------                     -----------
                             
99.1                            Press release dated October 19, 2004