Palladium and platinum nanoparticles were generated in the presence of a variety of protective polymers. The size and size distribution of the metal particles were determined by transmission electron microscopy, and the catalytic activities of the metal-polymer systems were qualitatively tested by the hydrogenation of cyclohexene as a model reaction. Several groups of protective polymers were examined, including water-soluble homopolymers and random copolymers. cationic polyelectrolytes, amphiphilic block copolymers, and latices as carriers. The catalytic properties were found to be significantly influenced by the type of protective polymer used.