The structure, morphology and composition of pure WO3 thin films deposited onto vacuum-cleaved NaCl(001) single crystals have been studied at different substrate temperatures up to 580 K and under different oxidative and reductive treatments in the temperature range 373–873 K by Transmission Electron Microscopy, Selected-Area Electron Diffraction and X-ray Photoelectron Spectroscopy (XPS). A transition from an amorphous structure obtained after deposition at 298 K to a more porous structure with small crystallites at the highest substrate temperatures has been observed. XPS spectra reveal the presence of W6+ irrespective of the preparation procedure. Significant changes in the film structure were only observed after an oxidative treatment in 1 bar O2 at 673 K, which induces crystallization of a monoclinic WO3 structure. After raising the oxidation temperature to 773 K, the film shows additional reconstruction and a hexagonal WO3 structure becomes predominant. This hexagonal structure persists at least up to 873 K oxidation temperature. However, these structural transformations observed upon oxidation were almost completely suppressed by mixing the WO3 thin film with a second oxide, e.g. Ga2O3. Reduction of the WO3 films in 1 bar H2 at 723–773 K eventually induced the formation of the β-W metal structure, as evidenced by electron diffraction and XPS.