Although both H-2 and CO can be thermodynamically oxidized by O-2 over TiO2 under ultraviolet light (UV) irradiation, it was found that CO not H-2 could be oxidized over an anatase TiO2 in this work. The chemisorption results of CO and H-2 at TiO2 surface under UV irradiation, investigated by a gas sensing testing, showed that CO adsorption at TiO2 would cause the decrease of TiO2 surface impedance, where as H-2 adsorption would cause its increase. It is proposed that the CO adsorbed at TiO2 donate electrons toTiO(2) (as a process of CO pre-oxidation), resulting in its oxidation. In contrast, the H-2 adsorbed at TiO2 accept electrons from TiO2 (as a process of H-2 pre-reduction), which makes it difficult to be oxidized. This result indicates that the photocatalytic oxidation of a reactant over TiO2 not only depends on the formation of the photo-generated carriers and the subsequent activated oxidizing species, but may be also depends on the electron transfer behavior at the interface of the adsorbed reactant and TiO2. (C) 2015 Elsevier B.V. All rights reserved.