Compounds of formula (I): [where: R.sup.1 is a direct bond, oxygen, a group >CH.sub.2, sulphur, a group >C.dbd.O, a group-(CH.sub.2).sub.2-- or a group-N--R.sup.a, where R.sup.a is hydrogen or alkyl; R.sup.3, R.sup.4, R.sup.5 and R.sup.6 are hydrogen or substituents.alpha.; R.sup.8, R.sup.9, R.sup.10 and R.sup.11 are hydrogen, hydroxy or alkyl; or R.sup.9 and R.sup.11 are joined to form a fused ring system with the benzene rings to which they are attached; R.sup.7 is a direct bond, oxygen or a --CH.sub.2-group; p is 0 or 1 ; substituents.alpha. are: alkyl, alkoxy, alkenyl, halogen, nitrile, hydroxyl, aryl, aralkyl, aryloxy, aralkyloxy, arylalkenyl, cycloalkyl, carboxy, carboxyalkoxy, alkoxycarbonyl, aryloxycarbonyl, alkylcarbonyloxy, alkanesulphonyl, arenesulphonyl, alkanoyl or arylcarbonyl; n is 1 to 12; R.sup.12 is hydrogen, methyl or ethyl; A is a group --[O(CHR.sup.13CHR.sup.14).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.13CHR.sup.14).sub.a]--, where: one of R.sup.13 and R.sup.14 is hydrogen and the other is hydrogen, methyl or ethyl; a is 1 to 2 ; b is 4 to 5 ; Q is a residue of a polyhydroxy compound having from 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; and X.sup.- is an anion]; and esters thereof 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.