Compounds of the formula I ##STR1## in which R.sub.1 is, for example, C.sub.1 -C.sub.20 alkyl or phenyl-C.sub.1 -C.sub.6 alkyl which radicals are unsubstitutedor substituted; R.sub.2, R.sub.3 and R.sub.4 independently of one another are phenyl or biphenyl, which radicals are unsubstituted or substituted; wherein the sum of the Hammett .sigma. constants (.SIGMA..sub..sigma.) of the substitutents on the aromatic radicals R.sub.2, R.sub.3 and R.sub.4 is between +0.36 and +2.58; R.sub.8, R.sub.9, R.sub.14, R.sub.15 and R.sub.16 are, for example, C.sub.1 -C.sub.12 alkyl, phenyl-C.sub.1 -C.sub.6 alkyl or phenyl, and G is a radical which is able to form positive ions. These compounds are suitable as photoinitiators for photopolymerizable compositions which contain acid groups, in the presence of a coinitiator if desired.