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The chemical composition of surface and underneath layers of WO3 thin films, deposited by thermal evaporation and annealed in air at different temperatures, has been studied by means of soft X-ray and X-ray photoemission spectroscopies. Both the W 4f and valence band spectra have been analyzed. The analysis has been performed on samples as inserted and after an annealing process in an ultra high vacuum. The results have shown that the surface always presents a nonstoichiometric WO3 compound, whose spectral components do not depend on the sample preparation. Instead, the study of the underneath layers has shown that the WO3 films annealed in air at 500°C are highly stoichiometric and stable, while the samples heated in air at 300°C are much more sensitive to the vacuum thermal treatment showing the presence of reduced WOx phases, whose intensity and chemical states change after the in vacuum annealing procedure.