The effect of tin on the hydrogenation of alpha -diketones over polymer-stabilized platinum nanoclusters (PVP-Pt) or the corresponding Al2O3 supported platinum nanoclusters (Pt/Al2O3) was investigated. It was shown that Sn2+ has a rate-promoting effect either on the enantioselective hydrogenation of alpha -diketones modified with cinchonidine or on the racemic hydrogenation of them. The average rate increases by a factor of more than 2 for the racemic hydrogenation of 3,4-hexanedione over polymer-stabilized platinum nanoclusters in ethanol solution with a Sn2+ concentration of 1.6 x 10(-4) mol/l. The enantioselectivity could also be enhanced by the addition of Sn2+ in the, enantioselective hydrogenation of these compounds. It is believed that the electrophilic nature of Sn2+ is responsible for the rate promotion.