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                            UNITED STATES                    OMB APPROVAL
                  SECURITIES AND EXCHANGE COMMISSION  --------------------------
                                                        OMB Number: 3235-0145
                       WASHINGTON, D.C. 20549         --------------------------
                                                      Expires: December 31, 2005
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                             SCHEDULE 13G             hours per response...11
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                    UNDER THE SECURITIES EXCHANGE ACT OF 1934
                                (AMENDMENT NO.)*


                              MKS Instruments, Inc.
- --------------------------------------------------------------------------------
                                (Name of Issuer)

                                  Common Stock
- --------------------------------------------------------------------------------
                         (Title of Class of Securities)

                                    55306N104
            --------------------------------------------------------
                                 (CUSIP Number)

                                   12/31/2004
- --------------------------------------------------------------------------------
             (Date of Event Which Requires Filing of this Statement)


Check the appropriate box to designate the rule pursuant to which this Schedule
is filed:

   /X/       Rule 13d-1(b)

   / /       Rule 13d-1(c)

   / /       Rule 13d-1(d)


*The remainder of this cover page shall be filled out for a reporting
person's initial filing on this form with respect to the subject class of
securities, and for any subsequent amendment containing information which
would alter the disclosures provided in a prior cover page.

The information required in the remainder of this cover page shall not be
deemed to be "filed" for the purpose of Section 18 of the Securities Exchange
Act of 1934 ("Act") or otherwise subject to the liabilities of that section
of the Act but shall be subject to all other provisions of the Act (however,
see the Notes).




CUSIP No. 55306N104
         --------------


- -------------------------------------------------------------------------------
  1. Names of Reporting Persons
     I.R.S. Identification Nos. of above persons (entities only).

          The TCW Group, Inc., on behalf of the TCW Business Unit
- -------------------------------------------------------------------------------
  2. Check the Appropriate Box if a Member of a Group (See Instructions)

                                                                       (a)  / /
                                                                       (b)  /X/
- -------------------------------------------------------------------------------
  3. SEC Use Only



- -------------------------------------------------------------------------------
  4. Citizenship or Place of Organization

          Nevada corporation

- -------------------------------------------------------------------------------
Number of Shares           5. Sole Voting Power
Beneficially Owned by
Each Reporting Person         -0-
With                       ----------------------------------------------------
                           6. Shared Voting Power

                              2,689,902
                           ----------------------------------------------------
                           7. Sole Dispositive Power

                              -0-
                           ----------------------------------------------------
                           8. Shared Dispositive Power

                              3,065,072
- -------------------------------------------------------------------------------
 9. Aggregate Amount Beneficially Owned by Each Reporting Person

         3,065,072

- -------------------------------------------------------------------------------
10. Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See
     Instructions)  / /

- -------------------------------------------------------------------------------
11. Percent of Class Represented by Amount in Row (9)

           5.7%(see response to Item 4)
- -------------------------------------------------------------------------------
12. Type of Reporting Person (See Instructions)

          HC/CO
- -------------------------------------------------------------------------------


ITEM 1.
     (a)   Name of Issuer

           MKS Instruments, Inc.
           --------------------------------------------------------------------
     (b)   Address of Issuer's Principal Executive Offices

           90 Industrial Way
           Wilmington, MA 01887
           --------------------------------------------------------------------

ITEM 2.
     (a)   Name of Person Filing
     (b)   Address of Principal Business Office or, if none, Residence
     (c)   Citizenship

           The TCW Group, Inc., on behalf of the TCW Business Unit
           865 South Figueroa Street
           Los Angeles, CA 90017
           (Nevada Corporation)
           --------------------------------------------------------------------
     (d)   Title of Class of Securities

           Common Stock
           --------------------------------------------------------------------
     (e)   CUSIP Number

           55306N104
           --------------------------------------------------------------------



ITEM 3.    IF THIS STATEMENT IS FILED PURSUANT TO SECTIONS 240.13d-1(b) or
           240.13d-2(b) or (c), CHECK WHETHER THE PERSON FILING IS A:

           (a)  / / Broker or dealer registered under section 15 of the
                    Act (15 U.S.C. 78o).

           (b)  / / Bank as defined in section 3(a)(6) of the Act
                    (15 U.S.C. 78c).

           (c)  / / Insurance company as defined in section 3(a)(19) of
                    the Act (15 U.S.C. 78c).

           (d)  / / Investment company registered under section 8 of the
                    Investment Company Act of 1940 (15 U.S.C. 80a-8).

           (e)  / / An investment adviser in accordance with Section
                    240.13d-1(b)(1)(ii)(E);

           (f)  / / An employee benefit plan or endowment fund in accordance
                    with Section 240.13d-1(b)(1)(ii)(F);

           (g)  /X/ A parent holding company or control person in accordance
                    with Section  240.13d-1(b)(1)(ii)(G);

                    (SEE Item 7)
                    The TCW Group, Inc., on behalf of the TCW Business Unit


           (h)  / / A savings association as defined in Section 3(b) of the
                    Federal Deposit Insurance Act (12 U.S.C. 1813);

           (i)  / / A church plan that is excluded from the definition of an
                    investment company under section 3(c)(14) of the
                    Investment Company Act of 1940 (15 U.S.C. 80a-3);

           (j)  / / Group, in accordance with Section 240.13d-1(b)(1)(ii)(J).
           If this statement is filed pursuant to Rule 13d-1(c), check this
           box. / /



ITEM 4.   OWNERSHIP **

           The TCW Group, Inc., on behalf of the Business Unit ***
           -------------------------------------------------------

     Provide the following information regarding the aggregate number and
percentage of the class of securities of the issuer identified in Item 1.

          (a)  Amount beneficially owned: 3,065,072
           --------------------------------------------------------------------
          (b)  Percent of Class:  5.7%

           --------------------------------------------------------------------
          (c)  Number of shares as to which the person has:

                  (i)   Sole power to vote or to direct the vote
                             none
                  -------------------------------------------------------------
                  (ii)  Shared power to vote or to direct the vote
                             2,689,902
                  -------------------------------------------------------------
                  (iii) Sole power to dispose or to direct the disposition of
                             none
                  -------------------------------------------------------------
                  (iv)  Shared power to dispose or to direct the disposition of
                             3,065,072
                  -------------------------------------------------------------

- -------------------
**    The filing of this Schedule 13G shall not be construed as an admission
      that the reporting person or any of its affiliates is, for the purposes of
      Section 13(d) or 13(g) of the Securities Exchange Act of 1934, the
      beneficial owner of any securities covered by this Schedule 13G.  In
      addition, the filing of this Schedule 13G shall not be construed as an
      admission that the reporting person or any of its affiliates is the
      beneficial owner of any securities covered by this Schedule 13G for any
      other purposes than Section 13(d) of the Securities Exchange Act of 1934.

***  See Exhibit A



ITEM 5.   OWNERSHIP OF FIVE PERCENT OR LESS OF A CLASS

          If this statement is being filed to report the fact that as of the
          date hereof the reporting person has ceased to be the beneficial owner
          of more than five percent of the class of securities, check the
          following / /.

ITEM 6.   OWNERSHIP OF MORE THAN FIVE PERCENT ON BEHALF OF ANOTHER PERSON

            Various persons other than as described in Item 4 have the right to
            receive or the power to direct the receipt of dividends from, or the
            proceeds from the sale of, the Common Stock of MKS Instruments, Inc.

ITEM 7.   IDENTIFICATION AND CLASSIFICATION OF THE SUBSIDIARY WHICH ACQUIRED
          THE SECURITY BEING REPORTED ON BY THE PARENT HOLDING COMPANY OR
          CONTROL PERSON

          SEE Exhibit A.

ITEM 8.   IDENTIFICATION AND CLASSIFICATION OF MEMBERS OF THE GROUP

          SEE Exhibit A.

ITEM 9.   NOTICE OF DISSOLUTION OF GROUP

          Not applicable.

ITEM 10.  CERTIFICATION

          Because this statement is filed pursuant to Rule 13d-1(b), the
          following certification is included:

          By signing below I certify that, to the best of my knowledge and
          belief, the securities referred to above were acquired in the
          ordinary course of business and were not acquired and are not held
          for the purpose of or with the effect of changing or influencing
          the control of the issuer of the securities and were not acquired
          and are not held in connection with or as a participant in any
          transaction having that purpose or effect.



                                    SIGNATURE

After reasonable inquiry and to the best of my knowledge and belief, I
certify that the information set forth in this statement is true, complete
and correct.

Dated this 9th day of February, 2005.

                                     The TCW Group, Inc., on behalf of the
                                     TCW Business Unit

                                     By: /s/ Linda D. Barker
                                         -------------------------------------
                                         Linda D. Barker
                                         Authorized Signatory



                                    EXHIBIT A


                 RELEVANT SUBSIDIARIES OF PARENT HOLDING COMPANY

PART A: TCW ENTITIES

PARENT HOLDING COMPANY:

      The TCW Group, Inc.



RELEVANT SUBSIDIARIES THAT ARE PERSONS DESCRIBED IN RULE 13d-1(b):

(i)      Trust Company of the West, a California corporation and a bank as
         defined in Section 3(a)(6) of the Securities Exchange Act of 1934.

(ii)     TCW Asset Management Company, a California corporation and an
         Investment Adviser registered under Section 203 of the Investment
         Advisers Act of 1940.

(iii)    TCW Investment Management Company, a California corporation and an
         Investment Adviser registered under Section 203 of the Investment
         Advisers Act of 1940.

This Schedule 13G is being filed by the TCW Group, Inc., a Nevada corporation
("TCW"), on behalf of itself and its direct and indirect subsidiaries, which
collectively constitute The TCW Group, Inc. business unit (the "TCW Business
Unit" or the "Reporting Person"). The TCW Business Unit is primarily engaged in
the provision of investment management services.

As of July 6, 2001, the ultimate parent company of TCW is Societe Generale,
S.A., a corporation formed under the laws of France ("SG"). The principal
business of SG is acting as a holding company for a global financial services
group, which includes certain distinct specialized business units that are
independently operated, including the TCW Business Unit.

SG, for purpose of the federal securities laws, may be deemed ultimately to
control TCW and the TCW Business Unit. SG, its executive officers and directors,
and its direct and indirect subsidiaries (including all business units except
the TCW Business Unit), may beneficially own shares of the securities of the
issuer to which this schedule relates (the "Shares") and such shares are not
reported in this statement. In accordance with Securities and Exchange
Commission ("SEC") Release No. 34-39538 (January 12, 1998), and due to the
separate management and independent operation of its business units, SG
disclaims beneficial ownership of Shares beneficially owned by the Reporting
Person. The Reporting Person disclaims beneficial ownership of Shares
beneficially owned by SG and any of SG's other business units.

                                       A-1