Effects of metal co-catalysts, Pd and Pt, on photocatalytic degradation of vinyl chloride monomer (VCM) were studied. Pd-photodeposited TiO2 (Pd-TiO2) did not produce any chlorinated organic compounds into the gas phase, while pure TiO2 produced monochloroacetylchloride and phosgene. The amounts of CO and formic acid formed by Pd-TiO2, were much smaller than those by TiO2, and the CO2 production proceeded selectively. However, Pd-TiO2 decomposed VCM more slowly and produced a larger amount of adsorptive chlorinated organic compound, as compared with TiO2. Photodeposited Pd is considered to inhibit the VCM decomposition pathway in which the gaseous byproducts are formed by the aid of Cl- ion adsorbed on the photocatalyst. The electron spin resonance (ESR) suggested that the photodeposited Pd suppress the formation of active species, such as Ti3+, O-2(-) and OH, which are responsible for the formation of gaseous byproducts. (C) 2002 Elsevier Science B.V. All rights reserved.