Monoclinic polycrystalline WO3 thin films with thickness of 150 nm have been obtained by evaporating high-purity WO3 powder onto Si(100) substrates and annealing in air at 300 and 500 °C for 24 h. The surface electronic properties of the thin films have been studied by angle integrated photoemission spectroscopy, using synchrotron radiation with photon energy ranging from 50 to 200 eV. Spectra have been measured of the W4f core levels and of the valence band (VB). The W4f line shape analysis has shown the presence of surface sub-stoichiometric WO3, confirming the presence of surface oxygen vacancies, ultimately responsible of the electronic transport properties and the gas sensitivity of the tungsten trioxide thin films. Correspondingly, the VB spectra show a well-defined W5d metallic peak near the Fermi edge. The samples have been also investigated after prolonged exposure to the ultraviolet beam and after submission to ultrahigh vacuum (UHV) annealing, monitoring variations in chemical surface composition (by observing the changes in the spectral line shapes). Photoemission results have shown the surface segregation of oxygen atoms desorbed from the bulk as a consequence of the UHV thermal treatments. This is in agreement with independent bulk sensitive photoemission and scanning tunnelling spectroscopy measurements.