Colloidal platinum nanoparticles were prepared by ethanol reduction of PtCl62- in the presence of poly(N-vinylformamide) (PNVF), poly(N-vinylacetamide) (PNVA), or poly(N-vinylisobutyramide) (PNVIBA). The effects of molecular weight and molar ratio of monomeric unit/Pt on the particle sizes and size distributions were characterized by transmission electron microscopy and UV-visible spectroscopy. The flocculation behavior of the polymer-stabilized colloidal Pt nanoparticles was studied with respect to the effects of temperature, addition of inorganic salts, and composition of the mixed solvents. The dispersion stability of the platinum colloids stabilized by poly(N'-isopropylacrylamide) (PNIPAAm) and poly(vinylpyrrolidone) (PVP) was also examined. The sequence of polymer-stabilized platinum colloids in increasing order of dispersion stability was found to be PNIPAAm-Pt < PNVIBA-Pt < PVP-Pt < PNVA-Pt < PNVF-Pt. Anions of the added salts played an essential role in reducing the dispersion stability and sulfates were more efficient at promoting flocculation than chlorides. The PNVF-Pt colloids did not show any critical flocculation point (temperature or salt concentration) under employed conditions, owing to the strongly hydrophilic nature of PNVF chains. The catalytic activity of PNVF-Pt colloids for hydrogenation of allyl alcohol in 0.8 M Na2SO4 solution was the same as that in pure water.