Compounds of the formulae I, II, III, IV and V wherein R1 i.a. is C4-C9cycloalkanoyl, C1-C12alkanoyl, C4-C6alkenoyl, or benzoyl; R2 is for example phenyl, C1-C20alkyl, C3-C8cycloalkyl, C2-C20alkanoyl, or benzoyl; Ar1 is R4S-phenyl or NR5R6-phenyl, each of which optionally is substituted; or Ar1 i.a. is optionally substituted; or Ar1 is naphthyl or anthracyl each of which is unsubstituted or substituted; or Ar1 is benzoyl, naphthalenecarbonyl, phenanthrenecarbonyl, anthracenecarbonyl or pyrenecarbonyl, each of which is unsubstituted or substituted, or Ar1 is 3,4,5-trimethoxyphenyl, phenoxyphenyl or biphenyl; Ar2 i.a. is optionally substituted, or naphthyl or anthracyl, each of which is unsubstituted or substituted, x is 2 or 3; M1 when x is 2, for example is phenylene, naphthalene, anthracylene, each of which optionally is substituted; M1, when x is 3, is a trivalent radical; M2 for example is M3 is for example C1-C12alkylene, cyclohexylene, or phenylene; n is 1-20; R3 is for example hydrogen or C1-C12alkyl; R3′ i.a. is C1-C12alkyl; substituted or —O-interrupted C2-C6alkyl; R4 is for example hydrogen, or C1-C12alkyl; and R5 and R6 independently of each other i.a. are hydrogen, C1-C12alkyl, or phenyl; are suitable as photoinitiators in particular in resist applications.