New covalently linked sulfonic acid modified silicas have been formed and characterised and their catalytic Bronsted activity evaluated. Thus phenylmercaptoethyltrimethoxysilane, (MeO)(3)SiCH2CH2SC6H5 and benzylmercaptoethyltrimethoxysilane, (MeO)(3)SiCH2CH2SCH2C6H5, were used to prepare the corresponding T-functional silica derivatives silica similar to CH2CH2SC6H4-4-SO3H and silica similar to 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. (C) 2007 Elsevier B.V. All rights reserved.