Compounds of the formula I or II wherein R1 is C1-C10haloalkylsulfonyl, halobenzenesulfonyl, C2-C10haloalkanoyl, halobenzoyl; R2 is halogen or C1-C10haloalkyl; Ar1 is phenyl, biphenylyl, fluorenyl, naphthyl, anthracyl, phenanthryl, or heteroaryl, all of which are optionally substituted; Ar′1 is for example phenylene, naphthylene, diphenylene, heteroarylene, oxydiphenylene, phenylene-D-D1-D-phenylene or —Ar″1-A1-Y1-A1-Ar″1—; wherein these radicals optionally are substituted; Ar″1 is phenylene, naphthylene, anthracylene, phenanthrylene, or heteroarylene, all optionally substituted; A1 is for example a direct bond, —O—, —S—, or —NR6—; Y1 inter alia is C1-C18alkylene; X is halogen; D is for example —O—, —S— or —NR6—; D1 inter alia is C1-C18alkylene; are particularly suitable as photolatent acids in ArF resist technology.