Several palladium and platinum nanocatalysts protected by cationic polyelectrolytes were prepared by the in-situ reduction of palladium chloride, PdCl2, and dihydrogen hexachloroplatinate, H2PtCl6. The particle sizes and size distributions were determined by transmission electron microscopy, and the colloids were further characterized by UV-vis spectroscopy. The catalytic activity of these nanoparticles was qualitatively investigated by the hydrogenation and conversion of cyclohexene as a model reaction and compared to palladium and platinum colloids protected by a selection of water-soluble, nonionic polymers. The results show that the catalytic activity is strongly influenced by the protective polymer chosen, as well as particle size and morphology. The use of cationic polyelectrolytes decreases the catalytic activities significantly, in comparison to several water-soluble, nonionic polymers investigated. The effects depend strongly on the particular metal, as illustrated in this case by differences observed between palladium and platinum.