Compounds of the formula I are suitable photoinitiators which accumulates at the surface of the formulation ##STR00001## in which R is for example phenyl and A is a surface-active radical of the formula III ##STR00002## Y is a divalent group C.sub.3 C.sub.12cycloalkylene, C.sub.6 C.sub.12bicycloalkylene; C.sub.1 C.sub.6alkylenen-C.sub.3 C.sub.12cycloalkylene, C.sub.1 C.sub.6alkylene- C.sub.6 C.sub.12bicycloalkylene or C.sub.1 C.sub.10alkylene interrupted by one or more non-consecutive C.sub.3 C.sub.12cycloalkylene, --U--C.sub.3 C.sub.12cycloalkylene, C.sub.6 C.sub.12bicycloalkylene or --U--C.sub.6 C.sub.12bicycloalkylene; C.sub.1 C.sub.10alkylene interrupted by one or more non-consecutive O and C.sub.3 C.sub.12cycloalkylene, --U--C.sub.3 C.sub.12cycloalkylene, C.sub.6 C.sub.12bicycloalkylene and/or --U--C.sub.6 C.sub.12bicycloalkylene; ##STR00003## U is U' or U'' U' is --(CH.sub.2).sub.aCOO--; U'' is --Si(CH.sub.3)(CH.sub.3)-- or --(CH.sub.2).sub.a--Si(CH.sub.3)(CH.sub.3)--, a is a number from 1 to 10; b is a number from 0 to 10; with the proviso that they are, however, at least 1 if the methylene group in question is between two oxygen atoms.