WO3 films with thickness of 80 nm have been thermally evaporated onto Si3N4/Si substrates. The films have been initially treated in oxygen by a 24-h-long annealing at 300 degreesC and 500 degreesC. XPS measurements, to follow W 4f. O 1s peaks and the valence band, have been performed on these samples as prepared and after successive ultra high vacuum (UHV) thermal treatments, We observed that the UHV annealing procedure produces a lack of oxygen at the surface of the as-deposited and 300 degreesC annealed samples strongly modifying the W 4f peak and producing the increase of metallic states at the Fermi edge, while very few modifications have been observed in the 500 degreesC sample. The films submitted to UHV thermal treatments have also been investigated as resistive gas sensors towards NO2. We observed a lowering of the base resistance and a decrease of the sensitivity properties with respect to the corresponding non-vacuum treated samples.