New covalently linked sulfonic acid modified silicas have been formed and characterised and their catalytic Bronsted activity evaluated. Thus phenylmercaptoethyltrimethoxysilane, (MeO)3SiCH2CH2SC6H5 and benzylmercaptoethyltrimethoxysilane, (MeO)3SiCH2CH2SCH2C6H5, were used to prepare the corresponding T-functional silica derivatives silica ∼ CH2CH2SC6H4-4-SO3H and silica ∼ CH2CH2SCH2C6H4-4-SO3H by grafting on commercial silica, followed by sulfonation of the aryl group. Solid-state NMR, DTA and surface area characteristics of the materials are reported. The mercaptophenylsulfonic acid materials are thermally robust highly efficient solid catalysts for a range of transformations including, esterification, etherification and carbonyl group protection. An apparent pore size effect on rate of conversion was observed. The mercaptobenzylsulfonic acid materials were comparatively unstable.