Abstract: Compounds of the formulae I, II, III and IV ##STR1## wherein Ar for example is a phenyl, biphenyl or benzoylphenyl group, which is unsubstituted or substituted; Ar.sub.1, for example, has the same meanings as Ar; Ar.sub.2 is inter alia phenyl; X may be a direct bond; Y hydrogen, etc.; R.sub.1 and R.sub.2 for example C.sub.1 -C.sub.8 alkyl; R.sub.3 inter alia hydrogen or C.sub.1 -C.sub.12 alkyl; R.sub.4 is inter alia C.sub.1 -C.sub.12 alkyl; or R.sub.3 and R.sub.4 together are C.sub.3 -C.sub.7 alkylene; R.sub.5 is for example C.sub.1 -C.sub.6 alkylene; and Z is a divalent radical; provided that at least one of the radicals Ar, Ar.sub.1, Ar.sub.2, Ar.sub.3, R.sub.1, R.sub.2, R.sub.3, R.sub.4, R.sub.5 or Y is substituted by 1 to 5 SH groups, or provided that Y contains at least one --SS-- group, are photoinitiators for the polymerization of ethylenically unsaturated compounds.