Asymmetric compounds of formula (I): [in which: R 1 , R 2 and R 3 each represents a direct bond, an oxygen atom, a group >CH 2 , a sulphur atom, a group >C=O or a group (CH 2 ) 2 -; R 4 , R 5 , R 6 , R 7 , R 8 , R 9 , R 10 and R 11 each represents a hydrogen atom, an alkyl, alkoxy or alkenyl group, a halogen atom, a nitro group, a nitrile group, a hydroxyl group, an aryl group, an aralkyl or aralkyloxy group, an aryloxy group, an arylalkenyl group, a cycloalkyl group, a carboxy group, an alkoxycarbonyl or alkylcarbonyloxy, an aryloxycarbonyl group, an alkanesulphonyl group, an arenesulphonyl group, an alkanoyl group or an arylcarbonyl group and X - represents an anion]; where the ring systems identified as A and B are different from each other, are useful as cationic photoinitiators for use in radiation-curable coating compositions, such as printing inks and varnishes, and have the advantages that they are relatively odour-free on curing, and allow the photoinitiator to be tailored more precisely to the desired application.