Pt/TiO2 catalysts were prepared by a colloidal deposition method including the photochemical preparation of Pt nanoparticles protected by poly(N-vinyl-2-pyrrolidone) and their deposition by liquid-phase adsorption on TiO2. Pt/TiO2 catalysts prepared by the colloidal deposition method with oxidation and H-2 reduction treatment showed higher CO oxidation activity than those obtained by impregnation and photodeposition techniques. FTIR studies revealed the reactivity of CO species adsorbed on Pt sites of these catalysts. The higher activity was ascribable to higher dispersion and higher reactivity of CO adsorbed on Pt particles.