Polyvinylpyrrolidone stabilized Pt and Pd colloids with mean particle sizes ranging from 1.5 to 3.1 nm were prepared and are effective catalysts for the selective hydrogenation of polyunsaturated soybean oil at 35 degreesC and atmospheric pressure. The rate constants for the three stages of hydrogenation from linoleneate to linoleate to oleate and to stearate were determined and their ratios, which signify the selectivity were calculated. Pt and Pd catalysts show different rates of hydrogenation towards the double bonds. Pt catalyst has lower oleate selectivity leading to higher yields of saturated product but lower percentage of trans-isomer than the Pd catalyst at comparable mean particle size of 1.5 nm. The oleate selectivity increases while the linoleate selectivity as well as the activity decreases with an increase of the mean particle size. An intermediate trans, trans-linoleate was detected for the Pt catalyst with larger particle mean size. (C) 2002 Elsevier Science B.V. All rights reserved.