MKS INC — Director's Dealing 2011

Mar 17, 2011

SEC Form 4 — Statement of Changes in Beneficial Ownership

Issuer: MKS INSTRUMENTS INC (MKSI)
CIK: 0001049502
Period of Report: 2011-03-16

Reporting Person: STEWART WILLIAM D (VP, Vacuum Products)

Non-Derivative Transactions

Date	Security	Code	Shares	Price	A/D	Holdings After	Ownership
2011-03-16	Common Stock	M	1960	—	Acquired	7532	Direct
2011-03-16	Common Stock	M	1400	—	Acquired	8932	Direct
2011-03-16	Common Stock	F	1046	$28.51	Disposed	7886	Direct

Derivative Transactions

Date	Security	Exercise Price	Code	Shares	A/D	Expiration	Underlying	Ownership
2011-03-16	Restricted Stock Unit	$	M	1960	Disposed		Common Stock (1960)	Direct
2011-03-16	Restricted Stock Unit	$	M	1400	Disposed		Common Stock (1400)	Direct

Footnotes

F1: Each restricted stock unit represents the contingent right to receive one share of common stock of MKS Instruments, Inc.

F2: This transaction was effected pursuant to a Rule 10b5-1 trading plan previously adopted by the reporting person.

F3: These RSUs are subject to the achievement of performance criteria determined in the first year of the grant and thereafter vests in equal annual installments over three years.

F4: The RSUs vest in equal annual installments over three years.