Aromatic-aliphatic ketones of the formulae I, II, III or IV ##STR1## wherein n is 1 or 2, Ar is an aryl radical, R.sup.1 and R.sup.2 are monovalent aliphatic, cycloaliphatic or araliphatic radicals, R.sup.3 is a direct bond or a divalent organic radical, X is a hydroxyl or amino group or the monovalent etherification or silylation products thereof, and X' is a divalent amino, ether or silyloxy group, Y is a direct bond or CH.sub.2 and Z is O, S, SO.sub.2, CH.sub.2 or C(CH.sub.3).sub.2, are suitable sensitizers for the photopolymerization of unsaturated compounds and for the photochemical crosslinking of polyolefins. Some of these compounds are novel and can be obtained by methods analogous to those for obtaining the known compounds of this type.