The enantioselective hydrogenation of ethyl pyruvate to (R)-ethyl lactate has been used as a reaction probe of the surface environment of colloidal platinum stabilized in ethanol with poly(vinylpyrrolidone). The rates of hydrogenation and the rates at which the acceleration is induced upon addition of the modifier (cinchonidine) have been compared for the colloidal catalyst and a 1% Pt/Al2O3 catalyst of equal platinum particle size under identical conditions. The results show that the presence of the stabilizing polymer in the colloidal catalyst does not hinder access of the modifier molecule to the colloidal metal surface but that the polymer may, by adsorption at the metal surface, reduce the number of surface-modified sites available for the faster enantioselective hydrogenation.