The effects of nanosized Pt deposits on TiO2 particles on the photocatalytic oxidation of gaseous CO were investigated. Platinum was photodeposited directly on a TiO2-coated glass plate with a typical Pt loading of similar to3 wt.%. Platinum deposition on TiO2 greatly enhances the photooxidation rate of CO in the concentration range 30-500 ppmv, whereas naked TiO2 samples exhibit much slower oxidation rates. The degree of Pt-induced enhancement strongly depends on the kind of TiO2 substrate. Three commercial TiO2 samples (Degussa P25, ISK STS-01, Hombikat UV-100) were platinized and compared for their photoactivity of CO oxidation. Platinized Hombikat TiO2 shows the best activity among them. The photoconversion of CO to M is quantitative in the presence of O-2. The oxidation states of photodeposited Pt include both Pt-0 and (PtO)-O-II with the former species dominant. The photocatalytic CO oxidation rates and the amounts of Pt deposited on TiO2 surface show a close correlation. The CO photooxidation is strongly inhibited in the absence of O-2 or in the presence of alternative oxidants such as N2O and H2O. The addition of water vapor little affects the CO photooxidation kinetics in the presence of O-2. Hydroxyl radicals do not seem to play a significant role in CO photooxidation on Pt/TiO2. The main oxidants in CO photocatalytic oxidation seem to be produced from adsorbed O-2, The active oxygen species generated on illuminated Pt/TiO2 surface are stable enough to survive for several minutes even in a dark period, and take part in a successive photooxidation reaction. The photocatalytic oxidation kinetics was investigated in detail as a function of Pt loading, coated TiO2 mass, CO concentration, O-2 concentration, light intensity, additives, and humidity. (C) 2003 Elsevier Science B.V. All rights reserved.