Tungsten oxide (WO3) films prepared by the sol-gel spin coating technique from a solution of tungsten oxychloride (S WO3) and for comparison from tungsten complex precursor (D WO3). Both types of films were analyzed by ellipsometry, Fourier transform infrared spectroscopy (FT-IR), X-ray diffractometry (XRD), scanning electron microscopy (SEM) and cyclic voltammetry (CV). XRD studies showed that both films were amorphous. The refractive index and electrochromic coloration were found to depend an the solution chemistry. Both films showed low absorption and high transparency in the visible range. We found that the refractive index (n) value of S WO3 films was higher than that of D WO3. The n and k values were n = 1.96 and k = 5.6 x 10(-3) for S WO3 films and n = 1.89 and k = 2.5 x 10(-3) for D WO3 films, both at 550 nm. The electrochemical behavior was investigated in 1M LiClO4/propylene carbonate solution. The optical and electrochemical properties clearly indicate that both WO3 films exhibit electrochemical reversibility beyond 1600 cycles without a change in performance. Spectroelectrochemical measurements showed that D WO, films exhibit a higher absorbance in their colored state as compared with S WO3 films. (C) 1997 Elsevier Science S.A.