Tungsten trioxide (WO3) thin films were prepared by spray pyrolysis under various conditions using, for the first time, tungsten chloride (WCl6) dissolved in 50% ethanol-50% wafer. All the as-deposited films were amorphous and dark. When annealed, they became crystallized and transparent. The structure and the composition of these films, before and after annealing, were studied. The X-ray diffraction diagrams revealed that all the annealed films consisted of the monoclinic WO3 phase. The X-ray photoelectron spectroscopy studies of the VV,, doublet showed the W6+ oxidation state: for all the films. The electron probe microanalysis revealed an over, stoichiometry of oxygen for the unannealed films. The optical properties as well as the structure of the annealed, therefore transparent, films prepared at different spray rates were also determined. The optical gap was determined from reflection and transmission spectra, All annealed samples exhibited a reversible electrochromic effect in the presence of an acid electrolyte as demonstrated by cyclic voltammetry and optical transmission measurements.