Photoacid generators generate sulfonic acids of formula (1a) upon exposure to high-energy radiation. RC(.dbd.O)R.sup.1--COOCH(CF.sub.3)CF.sub.2SO.sub.3.sup.-H.sup.+ (1a) R is hydroxyl, alkyl, aryl, hetero-aryl, alkoxy, aryloxy or hetero-aryloxy, R.sup.1 is a divalent organic group which may have a heteroatom (O, N or S) containing substituent, or R.sup.1 may form a cyclic structure with R. The photoacid generators are compatible with resins and can control acid diffusion and are thus suited for use in chemically amplified resist compositions.