Photoacid generators generate sulfonic acids of formula (1a) upon exposure to high-energy radiation. RC(═O)R1—COOCH(CF3)CF2SO3−H+ (1a) R is hydroxyl, alkyl, aryl, hetero-aryl, alkoxy, aryloxy or hetero-aryloxy, R1 is a divalent organic group which may have a heteroatom (O, N or S) containing substituent, or R1 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.