Photoinitiator compounds of formula (I): ##STR00001## {where: A represents a direct bond or a group of formula --[O(CHR.sup.7CHR.sup.6).sub.a].sub.y--, --[O(CH.sub.2).sub.bCO].sub.y--, or --[O(CH.sub.2).sub.bCO].sub.(y-1)--[O(CHR.sup.7CHR.sup.6).sub.a]--, where one of R.sup.6 and R.sup.7 is hydrogen and the other is hydrogen or methyl; a is a number from 1 to 2; b is a number from 4 to 5; Q is a residue of a polyhydroxy compound having 2 to 6 hydroxy groups; x is a number greater than 1 but no greater than the number of available hydroxyl groups in Q; y is a number from 1 to 10; R.sup.1, R.sup.2, R.sup.3 and R.sup.4 are individually the same or different and each is hydrogen, hydroxy, or alkyl; or R.sup.1 and R.sup.3 are joined to form a fused ring system with the benzene rings to which they are attached; and R.sup.5 represents a direct bond, an oxygen atom or a methylene group} are useful as cationic photoinitiators, especially for use in surface coating applications, such as printing inks and varnishes, and which are intended to be cured by polymerisation initiated by radiation.