Polyvinylpyrolidone stabilized Pt colloids with different polymer: Pt ratios with average particle sizes of between 1.4 and 2.9 nm and narrow size distribution were found to be active for the hydrogenation of palm olein at 35'C and atmospheric pressure. The initial hydrogen consumption rate was high, corresponding to the hydrogenation of the linoleate and slowed down thereafter, corresponding to the oleate hydrogenation. The rate constants for the two competing reactions were determined and the selectivity calculated to be between 10 and 100. The particle size of the Pt clusters influences the rate of the diene much more significantly than the monoene. The smaller the average particle size, the higher is the rate for the diene and hence the higher the selectivity. The trans-isomer formation was comparative low and was found to be a consequence of the partial saturation of the diene rather than the isomerization of the monoene.