Infrared spectra are reported of (CO)-C-12 and (CO)-C-12/(CO)-C-13 mixtures adsorbed on colloidal platinum with a mean particle size of 15 Angstrom, stabilized in liquid dispersion by polyvinylpyrrolidone. A shift. to higher frequency with increasing coverage of (CO)-C-12 is observed in the high-frequency (on-top) CO band, and for (CO)-C-12/(CO)-C-13 mixtures at maximum coverage a shift of frequencies of the two components of the on-top CO stretching mode is observed with changing (CO)-C-12/(CO)-C-13 ratio. These results are due to the presence of vibrational coupling between adsorbed CO molecules on the colloidal platinum particles. The correspondence between these observations and similar results on supported platinum particles and platinum single-crystal faces suggests that, even in the presence of solvent and dissolved polymer, the colloidal metal particles can react with CO to give adsorbed overlayers with significant similarity to those resulting from CO adsorption on clean metal surfaces.