Platinum colloids prepared by reduction of H2PtCl6 with methanol in the presence of polyvinylpyrrolidone have been examined in the sol state and following immobilisation on oxide supports. Both the sol and the supported catalysts contained platinum metal particles of 3-4 nm number-mean diameter, as determined by TEM. High initial activities for hydrogenation of the medium-sized alkenes trans-stilbene and methylcinnamate were recorded. The platinum particles were compared for particle size distribution and catalytic activity in the unsupported (sol) state and in supported states, both before and after heat treatment in oxygen-argon at 300 degrees C (calcination). Highest activities were measured for the sol state and for the calcined supported catalysts. It is suggested that this behaviour is related to the influence of support and polymer on the accessibility of the metal surface for the reactants, Marked activity differences between a platinum-colloid catalyst immobilised on a titania aerogel and a binary Pt-titania aerogel, directly prepared by a one-step sol-gel route with ensuing supercritical drying, are discussed.