Compounds of the formula I, II, III and IIIa ##STR1## in which Ar.sup.1 is an unsubstituted or substituted aromatic radical and at least one of the radicals R.sup.1 and R.sup.2 is an alkenyl, cycloalkenyl or arylmethyl group, are effective photoinitiators for photopolymerization of unsaturated compounds. They are particularly suitable for photocuring of pigmented systems.

Compounds of the formula I, II, III and IIIa ##STR1## in which Ar.sup.1 is an unsubstituted or substituted aromatic radical and at least one of the radicals R.sup.1 and R.sup.2 is an alkenyl, cycloalkenyl or arylmethyl group, are effective photoinitiators for photopolymerization of unsaturated compounds. They are particularly suitable for photocuring of pigmented systems.