The catalytic behaviour of some compounds containing C=O and/or C=C bonds has been studied over silica-supported platinum-based catalysts, modified with tin. Tin was introduced by means of surface organometallic chemistry on metals (SOMC/M) techniques. The effect of the obtention conditions upon the catalytic performance was evidenced through the study of three systems having the same Sn/Pt atomic ratio (0.4), but prepared and activated at different temperature. In the hydrogenation of butyraldehyde and butanone, the adsorption of the eta(1)-(O) type appears as highly favourable, both from a geometric and electronic point of view. In the benzaldehyde hydrogenation, the increase in the catalytic activity for PtSn-OM and PtSn-BM systems is quite more important than in the PtSn-OM* system, fundamentally by electronic effects associated with the presence of ionic tin and of the phenyl group. In the case of the cyclohexene, geometric and electronic, as well as steric effects lead to a strong reduction of the hydrogenation rate of C=C bond. These results can be extrapolated to explain the behaviour of the unsaturated alpha,beta-aldehydes. The hydrogenation of the C=O group is promoted and the adsorption modes favourable to the C=C hydrogenation are inhibited by tin. The combination of both effects leads to the sequence of selectivity to UOL: Pt much less than PtSn-OM* < PtSn-BM < PtSn-OM. (C) 2002 Elsevier Science B.V. All rights reserved.