A preferred group of ultraviolet stabilizing compounds of the formula I are those of the formula Ia, ##STR3## in which each of R1 ' and R1 ', independently, is a C1-8 alkyl radical or a phenyl group, unsubstituted or substituted by up to 4 substituents selected from a hydroxyl group, a halogen atom and 1 or 2 C1-4 alkyl radicals,