Pt/TiO2 was active for the oxidation of CO to CO2 in the dark at ambient temperature after UV irradiation. Pure TiO2 was inactive and the presence of Pt supported on TiO2 was indispensable for the reaction. Based on the ESR study, it was found that Pt deposited on TiO2 stabilized the O- and O-3(-) species photoformed on the TiO2 and they were responsible for the CO oxidation.