The thermal properties of vanadium pentoxide (V2O5) thin films have been studied by X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). XPS and UPS data demonstrate that V2O5 thin films are gradually reduced by annealing in the ultrahigh vacuum chamber at temperatures up to 400 degreesC due to the formation of oxygen vacancy. The oxygen defect in the remaining thin film leads to the appearance of a new emission line at about 10.3 eV in the valence bands, which is direct evidence for oxygen vacancy on a solid surface. (C) 2004 Elsevier B.V. All rights reserved.