Peroxy-benzoic acid residues have been attached to polystyrene resins with nominal crosslink ratios of 5-40Β°o, and also to various commercially available silicas and aluminas. Chemical modification of the resins becomes increasingly difficult as the crosslink ratio is increased, although resistance to oxidative degradation by hydrogen peroxide in methanesulphonic acid is simultaneously enhanced. Two different linkages were employed for the binding of peroxy-acid groups to the inorganic oxide supports, one involving a direct silicon aromatic bond and the other with an ethylene bridge interposed. In both cases, the introduction of peroxy-acid groups proceeded with ease and without any apparent degradation although the Ioadings achieved were somewhat lower than with the resins, as anticipated. All the supported peroxyacids functioned as oxidants for tetrahydrothiophene, the 5Β°, crosslinked species being the most effective of the resin bound acids. One macroreticular resin displayed partial selectivity in oxidizing the thioether to its sulphoxide rather than sulphone. The best of the inorganic oxide supported oxidants approached closely to the reactivity shown by the 5Β°0 crosslinked resin; a mixed silica/alumina support showed the highest selectivity for sulphoxide formation in the case when its bound peroxy-acid was attached by a direct silicon aromatic bond.