SEC Form 4

FORM 4		UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934 or Section 30(h) of the Investment Company Act of 1940	OMB APPROVAL OMB Number: 3235-0287 Estimated average burden hours per response: 0.5
	Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).

1. Name and Address of Reporting Person* STEWART WILLIAM D (Last) (First) (Middle) 2 TECH DRIVE (Street) ANDOVER MA 01810 (City) (State) (Zip)	2. Issuer Name and Ticker or Trading Symbol MKS INSTRUMENTS INC [ MKSI ]	5. Relationship of Reporting Person(s) to Issuer (Check all applicable) Director 10% Owner X Officer (give title below) Other (specify below) V.P. Vacuum Products & PFMC
	3. Date of Earliest Transaction (Month/Day/Year) 05/19/2008
	4. If Amendment, Date of Original Filed (Month/Day/Year) 05/20/2008	6. Individual or Joint/Group Filing (Check Applicable Line) X Form filed by One Reporting Person Form filed by More than One Reporting Person

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned
1. Title of Security (Instr. 3)	2. Transaction Date (Month/Day/Year)	2A. Deemed Execution Date, if any (Month/Day/Year)	3. Transaction Code (Instr. 8)		4. Securities Acquired (A) or Disposed Of (D) (Instr. 3, 4 and 5)			5. Amount of Securities Beneficially Owned Following Reported Transaction(s) (Instr. 3 and 4)	6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4)	7. Nature of Indirect Beneficial Ownership (Instr. 4)
			Code	V	Amount	(A) or (D)	Price
Common Stock(1)	05/19/2008		S		3,102	D	$25.72	12,000	D

Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned (e.g., puts, calls, warrants, options, convertible securities)
1. Title of Derivative Security (Instr. 3)	2. Conversion or Exercise Price of Derivative Security	3. Transaction Date (Month/Day/Year)	3A. Deemed Execution Date, if any (Month/Day/Year)	4. Transaction Code (Instr. 8)		5. Number of Derivative Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5)		6. Date Exercisable and Expiration Date (Month/Day/Year)		7. Title and Amount of Securities Underlying Derivative Security (Instr. 3 and 4)		8. Price of Derivative Security (Instr. 5)	9. Number of derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4)	10. Ownership Form: Direct (D) or Indirect (I) (Instr. 4)	11. Nature of Indirect Beneficial Ownership (Instr. 4)
				Code	V	(A)	(D)	Date Exercisable	Expiration Date	Title	Amount or Number of Shares

Explanation of Responses:

1. This sale was unintentionally omitted from the previous Form 4 filing on May 20, 2008

	/s/Renee M. Donlan POA	02/18/2009
	** Signature of Reporting Person	Date
Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.
* If the form is filed by more than one reporting person, see Instruction 4 (b)(v).
** Intentional misstatements or omissions of facts constitute Federal Criminal Violations See 18 U.S.C. 1001 and 15 U.S.C. 78ff(a).
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POWER OF ATTORNEY

Know all by these presents, that the undersigned hereby constitutes
and appoints each of  Kathleen F. Burke and Renee M. Donlan, signing
singly / jointly, the undersigned's true and lawful attorney in fact
to:

(1)  execute for and on behalf of the undersigned, in the undersigned's
capacity as an officer and/or director of MKS Instruments Inc. (the
Company), Forms 3, 4,  5 and 144 in accordance with Section 16(a) of
the Securities Exchange Act of 1934 and the rules thereunder;

(2)  do and perform any and all acts for and on behalf of the
undersigned which may be necessary or desirable to complete and execute
any such Forms, complete and execute any amendment or amendments thereto,
and timely file such form with the United States Securities and Exchange
Commission and any stock exchange or similar authority; and

(3)  take any other action of any type whatsoever in connection with the
foregoing which, in the opinion of such attorney in fact, may be of
benefit to, in the best interest of, or legally required by, the
undersigned, it being understood that the documents executed by such
attorney in fact on behalf of the undersigned pursuant to this Power of
Attorney shall be in such form and shall contain such terms and
conditions as such attorney in fact may approve in such attorney in
fact's discretion.

The undersigned hereby grants to each such attorney in fact full power
and authority to do and perform any and every act and thing whatsoever
requisite, necessary, or proper to be done in the exercise of any of the
rights and powers herein granted, as fully to all intents and purposes
as the undersigned might or could do if personally present, with full
power of substitution or revocation, hereby ratifying and confirming
all that such attorney in fact, or such attorney in fact's substitute
or substitutes, shall lawfully do or cause to be done by virtue of this
power of attorney and the rights and powers herein granted.  The
ndersigned acknowledges that the foregoing attorneys in fact, in
serving in such capacity at the request of the undersigned, are not
assuming, nor is the Company assuming, any of the undersigned's
responsibilities to comply with Section 16 of the Securities Exchange
Act of 1934.

This Power of Attorney shall remain in full force and effect until the
undersigned is no longer required to file Forms 3, 4, 5 and 144 with
respect to the undersigned's holdings of and transactions in securities
issued by the Company, unless earlier revoked by the undersigned in a
signed writing delivered to the foregoing attorneys in fact.

IN WITNESS WHEREOF, the undersigned has caused this Power of Attorney
to be executed as of this 10th day of February, 2009.

/s/ William Stewart
____________________________
Signature