Several colloidal palladium nanocatalysts prepared by the in, situ reduction of palladium chloride PdCl2, ammonium tetrachloropalladate (NH4)(2)PdCl4, and palladium acetate Pd(CH3COO)(2) were protected by various water-soluble polymers, with special emphasis on polyacids. The particle sizes, morphologies, and size distributions of the palladium nanoparticles were determined by transmission electron microscopy (TEM), and their catalytic activities were qualitatively tested by the hydrogenation of cyclohexene. The type of the polymer (for example, polyacid versus a nonionic, water-soluble polymer) can influence the nanoparticle sizes and morphologies, as well as colloidal stabilities. For the catalytic activities of these metal-polymer systems, the choice of the protective polymer can be equally important. Lower catalytic activities have been mostly found if polyacids were used as protective matrices for these palladium nanocatalysts. It was found to be important to consider several influences, such as the particle size and morphology, as well as the interaction between the polymer and the catalyst nanoparticle. Thus, the selection of the protective polymer is crucial for the development of tailored metal-polymer catalyst systems. Additional influences may stem from the presence of ions, for example, those from the metal precursor, or the counterions of the polymer side groups.