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Tungsten trioxide (WO3) thin films were prepared by spray pyrolysis. The obtained films were amorphous and dark-brown. The films were subsequently annealed at 500°C in an air atmosphere for different annealing times. The IR measurements revealed the presence of water in our films and enabled us to follow its evolution as a function of the annealing conditions. The composition of the films was investigated by X-ray photoelectron spectroscopy. W 4f and O 1s core level peaks were decomposed and all the hidden peaks were identified. The XRD study showed that the crystallinity increased with the annealing time. The optical study indicated that the annealed films were transparent. The optical gap and the refractive index were found to be in agreement with data in the literature. Nevertheless, spectral measurements of the diffuse transmittance showed that this component was significant and varied with the annealing time.