An iodonium compound of formula (I): ##STR00001## where R.sup.AR1 is a C.sub.5-6 aryl group, bearing at least one substituent selected from formyl, thionoacyl, acylamidocarboxy, thionoester, azo, C.sub.2-20 alkenyl, C.sub.2-20 alkynyl, and (CH.sub.2).sub.nR.sup.C, where R.sup.C is selected from ether, amino, azo and thioether; R.sup.AR2 is a C.sub.5-10 aryl group, optionally substituted by one or more groups selected from C.sub.1-12 alkyl, C.sub.5-12 aryl, C.sub.3-12 heterocyclyl, ether, thioether, nitro, cyano and halo, and may be linked to a solid support or fluorous tag; and X is a counteranion.