Compounds of formula (I), in which X is oxygen or sulfur, R.sub.1, R.sub.2 and R.sub.3 independently of one another are a group (a), C.sub.2 -C.sub.8 alkenyl, phenyl which is unsubstituted or is substituted from one to four times with C.sub.1 -C.sub.12 alkyl, C.sub.1 -C.sub.12 alkoxy, C.sub.1 -C.sub.6 alkylthio and/or halogen, or are naphthyl which is unsubstituted or is substituted from one to four times with C.sub.1 -C.sub.12 alkyl, C.sub.1 -C.sub.12 alkoxy, C.sub.1 -C.sub.6 alkylthio and/or halogen, or are biphenyl which is unsubstituted or is substituted from one to four times with C.sub.1 -C.sub.12 alkyl, C.sub.1 -C.sub.12 alkoxy, C.sub.1 -C.sub.6 -alkylthio and/or halogen, or R.sub.1, R.sub.2 and R.sub.3 independently of one another are an O-, S- or N-containing 5- or 6-membered heterocyclic ring, R.sub.1 is in addition a group of formula (II), or R.sub.1 and R.sub.2 are linked to form a ring containing 4 to 10 carbon atoms which is unsubstituted or substituted by 1 to 6 C.sub.1 -C.sub.4 alkyl groups, R.sub.4, R.sub.5 and R.sub.6 independently of one another are hydrogen or C.sub.1 -C.sub.18 alkyl, or R.sub.5 and R.sub.6, together with the carbon atom to which they are attached, form a cyclopropyl, cyclopentyl or cyclohexyl ring, and R.sub.7, R.sub.8, R.sub.9 and R.sub.10 independently of one another are hydrogen, C.sub.1 -C.sub.4 alkyl, C.sub.1 -C.sub.4 alkoxy or halogen, are suitable as photoinitiators for the photopolymerization of ethylenically unsatured systems. ##STR1##