The heterogeneous photocatalytic degradation of perchloroethene (PCE) was studied in the gas phase, with TiO2 as photocatalyst. The adsorption of the substrate and the products was measured in the presence and absence of H2O. In the absence of H2O vapor, the Langmuir adsorption model describes the adsorption process well. H2O vapor decreases the amount of PCE adsorbed at lower concentration because of the competitive adsorption of PCE and H2O on the surface of the catalyst. In the presence of H2O vapor, the Langmuir model is not valid any more. The effects of the concentrations of the substrate, O-2 and H2O on the degradation of PCE were determined. On increase of the initial concentration of the substrate or O-2 in the gas mixture, the degradation rate of PCE was enhanced up to a certain level. The presence of H2O vapor decreased the rate of PCE degradation by decreasing the amount of PCE adsorbed on the surface of the catalyst. An induction period was found in the first stage of the decomposition. The curves of trichloroacetyl chloride and phosgene accumulations indicated this retardation effect, but the accumulation of CO2 was monotonous. The induction period is caused by the competitive adsorption of various compounds and the action of the trichloroacetyl chloride formed. On the basis of the present experimental results and previous evidence, a chemical mechanism involving reaction steps that are not generally used was proposed. (C) 2004 Elsevier B.V. All rights reserved.