Compounds of the formulae I, II, III, IV and V ##STR00001## wherein R.sub.1 i.a. is C.sub.4-C.sub.9cycloalkanoyl, C.sub.1-C.sub.12alkanoyl, C.sub.4-C.sub.6alkenoyl, or benzoyl; R.sub.2 is for example phenyl, C.sub.1-C.sub.20alkyl, C.sub.3-C.sub.8cycloalkyl, C.sub.2-C.sub.20alkanoyl, or benzoyl; Ar.sub.1 is R.sub.4S-phenyl or NR.sub.5R.sub.6-phenyl, each of which optionally is substituted; or Ar.sub.1 i.a. is ##STR00002## optionally substituted; or Ar.sub.1 is naphthyl or anthracyl each of which is unsubstituted or substituted; or Ar.sub.1 is benzoyl, naphthalenecarbonyl, phenanthrenecarbonyl, anthracenecarbonyl or pyrenecarbonyl, each of which is unsubstituted or substituted, or Ar.sub.1 is 3,4,5-trimethoxyphenyl, phenoxyphenyl or biphenyl; Ar.sub.2 i.a. is ##STR00003## optionally substituted, or naphthyl or anthracyl, each of which is unsubstituted or substituted, x is 2 or 3; M.sub.1 when x is 2, for example is phenylene, naphthalene, anthracylene, each of which optionally is substituted; M.sub.1, when x is 3, is a trivalent radical; M.sub.2 for example is ##STR00004## M.sub.3 is for example C.sub.1-C.sub.12alkylene, cyclohexylene, or phenylene; n is 1-20; R.sub.3 is for example hydrogen or C.sub.1-C.sub.12alkyl; R.sub.3' i.a. is C.sub.1-C.sub.12alkyl; substituted or --O-interrupted C.sub.2-C.sub.6alkyl; R.sub.4 is for example hydrogen, or C.sub.1-C.sub.12alkyl; and R.sub.5 and R.sub.6 independently of each other i.a. are hydrogen, C.sub.1-C.sub.12alkyl, or phenyl; are suitable as photoinitiators in particular in resist applications.