The effect of Cl-ions on photooxidation of propylene on TiO2 semiconductor was investigated. Cl-/TiO2 catalysts were prepared by annealing Degussa P25 TiO2 in the gas flow of N-2 and Cl-2 under 100-400 degrees C. The photocatalytic oxidation of propylene was carried out in a continuous flow system, with the chromatograph to analyze the products on line. The experimental results showed that the activity of Cl-/TiO2 catalysts increased as heat-treated temperature decreased. The activity of the sample heat-treated at 100 degrees Cwas about two times higher than that of pure TiO2. Moreover, as to TiO2, the main product of the propylene photocatalytic oxidation was CO2, but with Cl-/TiO2 catalysts, not only CO2 but also trace CO was determined. The adsorbed species on TiO2 surface before and after reaction were analyzed by X-ray ;photoelectron spectroscopy (XPS) and thermogravimetric/differential thermal analyses (TG-DTA) coupled to a mass spectrometer (MS). XPS analysis showed that there was Cl- absorbed on the Cl-/TiO2 surface, and the absorption amount of Cl- decreased after the photooxidation reaction of propylene. TGDTA-MS analysis confirmed chlorine absorbed on the surface of TiO2 in the form of Cl-ion. These results illuminated that absorbed Cl- on the surface of TiO2 formed a weak physical absorption on TiO2 at low temperature, and subsequently participated in the photooxidation of propylene, finally removed from TiO2 surface. (C) 2009 Elsevier B.V. All rights reserved.