Several stable colloidal platinum nanocatalysts were prepared by in-situ reduction of hexachloroplatinic acid H2PtCl6, and were protected by various water-soluble homopolymers and random copolymers as well as cationic polyelectrolytes. The particle sizes, morphologies, and size distributions were determined by transmission electron microscopy (TEM), and the catalytic activity of the platinum nanoparticles was tested by the hydrogenation of cyclohexene. The type of protective polymer and its properties strongly influence the catalytic activity by creating a certain "environment" surrounding the catalytically-active nanometal. Thus, careful selection of the protective polymer plays an important role in the development of tailored metal-polymer catalyst systems.