The electrochromic (EC) properties of tungsten oxide (WO3), such as coloration efficiency, cyclic durability and reversibility strongly depend on the structural and morphological properties, which are influenced by the deposition method and parameters. This paper presents the steps for optimizing the deposition parameters (substrate temperature, air flow pressure and precursor solution molarity) for improving the optical and electrical properties of WO3 thin films for EC applications. WO3 thin films were deposited by spray pyrolysis using tungsten hexachloride (WCl6) dissolved in ethanol as precursor solution. The EC properties of optimized films were tested in two different electrolytes (H2SO4 1 M and acetic acid/sodium acetate buffer with pH = 4) and changes in structure, composition and morphology of the films after coloration/bleaching cycles were discussed. The deposition temperature, carrier gas pressure and solution molarity were optimized at 250 degrees C, 120 kPa and 0.14 M respectively. Under these condition a dense, uniform film, with homogenous distribution of particles, good adhesion to the substrate, low roughness (9.02 nm), high transparency (>70% in the 500-1100 nm range) and conductivity was obtained. Transmission modulation is higher for the sample cycled in H2SO4 1 M (64% at 630 nm) compared to that cycled in the buffer (21% at 630 nm), whereas opposite results were obtained for coloration efficiencies 28 cm(2) C-1 (at 630 nm) and 35 cm(2) C-1 (at 630 nm), respectively. Changes in surface chemistry and morphology of the optimized sample were observed after cycling in H2SO4. (C) 2012 Elsevier B.V. All rights reserved.