In this work, the electronic properties of the surface of WO3 films with thickness of 150 run, thermally evaporated in high vacuum onto Si(100) substrates and pre-treated in air by a 24-h-long annealing at 300 degreesC and 500 degreesC (obtaining polycrystalline monoclinic samples) have been studied by surface and bulk sensitive core level (W 4f) and angle integrated valence band photoemission using synchrotron radiation (ELETTRA Synchrotron). The photon energy ranged from 50 eV to 200 ev The line shape analysis of W 4f core level spectra has shown that the surface presents a sub-stoichiometric WO3 component assigned to oxygen vacancies ultimately responsible for the gas sensitivity of this material. Correspondingly, valence band spectra show well-defined metallic states W 5d in the gap and near the Fermi level. The variations of surface chemical composition caused by Ultra High Vacuum annealing, and prolonged exposure to UV beam has been monitored by changes in spectral line shape. A general consequence of annealing in vacuum is the segregation of oxygen from the bulk toward the surface as confirmed by independent scanning tunnelling spectroscopy measurements. (C) 2003 Elsevier Science B.V. All rights reserved.