The catalytic oxidation of SO2 to SO3 was studied over platinum based catalysts in the absence and the presence of dopants. The active metal was supported on silica gel or titania (anatase) by impregnation. The activities of the silica supported catalysts were found to follow the order PtRh/SiO2 > Pt/SiO2 > Pt-Al/SiO2. For the samples supported on titania the respective order was Pt/TiO2 > Pt-Rh/TiO2 > Pt-Al/TiO2. The size of the particles of the active phase, the presence of dopants and the purity of the catalyst were found to affect the catalytic performance. A careful selection of the pH of the impregnation solution and of the reduction temperature of the precursor salts resulted in a very active catalyst with average particle size of 1.7 nm. (c) 2006 Elsevier B.V. All rights reserved.