Lactone compounds of formula (1) are useful as monomers to form base resins for use in chemically amplified resist compositions adapted for micropatterning lithography. R1 is H or C1-6 alkyl, R2 is H or an acyl or alkoxycarbonyl group of 1-15 carbon atoms which may be substituted with halogen atoms, Z is a divalent C1-15 organic group which forms a lactone ring with the carbonyloxy group, k is 0 or 1, and m is an integer from 0 to 5.