X0303 4 2011-05-19 0001049502 MKS INSTRUMENTS INC MKSI 0001202728 ANDERSON ROBERT R 2 TECH DRIVE ANDOVER MA 01810 1 Common Stock 2011-05-19 4 S 0 1953 26.03 D 21000 D This transaction was executed in multiple trades at prices ranging from $26.00 to $26.07 The price reported above reflects the weighted average sale price. The reporting person hereby undertakes to provide, upon request, the full information regarding the number of shares and prices at which the transaction was effected. /s/Renee M. Donlan POA 2011-05-22