Poly(N-vinyl-2-pyrrolidone) (PVP)-stabilized ruthenium colloids were prepared by NaBH4 reduction at room temperature and characterized by transmission electron microscopy and x-ray photoelectron spectra. The average diameters of the colloids were 1.3-1.8 nm with sigma = 0.4-0.7 mn with respect to the preparation conditions. The PVP-stabilized ruthenium colloids were used as catalysts in the hydrogenations of cyclooctene and n-heptene and the selective hydrogenation of citronellal to citronellol.