As initiators for the photopolymerisation of ethylenically unsaturated compounds, the use of systems consisting of compounds of benzophenone type of formula I ##STR1## in which R, R' and R.sup.2 are preferably alkyls of 1-4 carbon atoms, R.sup.3 and R.sup.4 are preferably hydrogen and Ar is preferably phenyl, in combination with donors of hydrogen or substances which determine the formation of labile photochemical complexes, such as alcohols, ethers and tertiary amines with hydrogen in the alpha position, and the use for the same purpose of mixtures of 2,4,6-trialkylbenzophenones and benzophenone and particularly of a mixture of 2,4,6-trimethylbenzophenone and benzophenone in a molar ratio of 1.155, which has the advantage of being liquid at temperatures exceeding 10.degree. C. and of possessing photochemical activity superior to that of benzophenone.