The invention pertain to novel photoacid generator compounds of the formula I, II or III ##STR00001## wherein R.sub.1 is for example C.sub.1-C.sub.18alkylsulfonyl or phenylsulfonyl, phenyl-C.sub.1-C.sub.3alkylsulfonyl, , naphthylsulfonyl, anthracylsulfonyl or phenanthrylsulfonyl, all optionally substituted, or R.sub.1 is a group ##STR00002## X.sub.1, X.sub.2 and X.sub.3 independently of each other are O or S; R'.sub.1, is e.g. phenylenedisulfonyl, naphthylenedisulfonyl, diphenylenedisulfonyl, or oxydiphenylenedisulfonyl, all optionally substituted; R.sub.2 is halogen or C.sub.1-C.sub.10haloalkyl; X is halogen; Ar.sub.1 is for example biphenylyl or fluorenyl, or is substituted naphthyl; Ar'.sub.1 is heteroarylene, optionally substituted; R.sub.8, R.sub.9, R.sub.10 and R.sub.11 for example are C.sub.1-C.sub.6alkyl which is unsubstituted or substituted by halogen; or R.sub.8, R.sub.9 and R.sub.10 are phenyl which is unsubstituted or substituted by C.sub.1-C.sub.4alkyl or halogen; or R.sub.10 and R.sub.11 together are 1,2-phenylene or C.sub.2-C.sub.6alkylene which is unsubstituted or substituted by C.sub.1-C.sub.4alkyl or halogen.