r disubstituted by C1 -C4 alkyl and/or C1 -C4 alkoxy with the provisos that at least one of the radicals R3, R4, R5, R6 and R7 is other than hydrogen and that, if R1 and R2 are methyl, R3 and R6 are not methyl, and mixtures of such compounds with ??-hydroxy ketones, benzophenones and ??-amino ketones, are suitable as photoinitiators.