The invention pertain to novel photoacid generator compounds of the formula I, II or III wherein R1 is for example C1-C18alkylsulfonyl or phenylsulfonyl, phenyl-C1-C3alkylsulfonyl, naphthylsulfonyl, anthracylsulfonyl or phenanthrylsulfonyl, all optionally substituted, or R1 is a group X1, X2 and X3 independently of each other are O or S; R′1, is e.g. phenylenedisulfonyl, naphthylenedisulfonyl, diphenylenedisulfonyl, or oxydiphenylenedisulfonyl, all optionally substituted; R2 is halogen or C1-C10haloalkyl; X is halogen; Ar1 is for example biphenylyl or fluorenyl, or is substituted naphthyl; Ar′1 is heteroarylene, optionally substituted; R8, R9, R10 and R11 for example are C1-C6alkyl which is unsubstituted or substituted by halogen; or R8, R9 and R10 are phenyl which is unsubstituted or substituted by C1-C4alkyl or halogen; or R10 and R11 together are 1,2-phenylene or C2-C6alkylene which is unsubstituted or substituted by C1-C4alkyl or halogen.