Compounds of formula I, II and III, wherein wherein R1 is for example hydrogen, C1-C12alkyl, C3-C30cycloalkyl, C2-C12alkenyl, C4-C8cycloalkenyl, phenyl, which is unsubstituted or substituted, naphthyl, anthracyl or phenanthryl, unsubstituted or substituted, heteroaryl radical which is unsubstituted or substituted; wherein all radicals R1 with the exception of hydrogen can additionally be substituted by a group having a —O—C-bond or a —O—Si-bond which cleaves upon the action of an acid; R′1 is for example phenylene, naphthylene, diphenylene or oxydiphenylene, wherein these radicals are unsubstituted or substituted; R2 is halogen or C1-C10haloalkyl; R3 is for example C1-C18alkylsulfonyl, phenylsulfonyl, naphthylsulfonyl, anthracylsulfonyl or phenanthrylsulfonyl, wherein the groups are unsubstituted or substituted, or R3 is e.g. C2-C6haloalkanoyl, or halobenzoyl, R′3 is for example phenylenedisulfonyl, naphthylenedisulfonyl, diphenylenedisulfonyl, or oxydiphenylenedisulfonyl, wherein these radicals are unsubstituted or substituted, X is halogen; are especially suitable as photosensitive acid-donors in chemically amplified resist formulations.