Compounds of formula (I): ##STR00001## [in which: R.sup.1 represents a direct bond, an oxygen atom, a group >CH.sub.2, a sulphur atom, a group >C.dbd.O, a group --(CH.sub.2).sub.2-- or a group of formula --N--R.sup.a, where R.sup.a is hydrogen or an alkyl; R.sup.4, R.sup.5, R.sup.6 and R.sup.7 are each hydrogen or various groups or atoms; R.sup.8, R.sup.9, R.sup.10 and R.sup.11 are each hydrogen, hydroxy, or alkyl; or R.sup.9 and R.sup.11 form a fused ring system with the benzene rings to which they are attached; R.sup.12 is a direct bond, oxygen atom or methylene; 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.