Tungsten oxide (WO3) thin films are prepared by using a simple and inexpensive solution thermolysis technique. Thin film samples of different thickness are obtained by varying quantity of ammonium tungstate solution sprayed onto the preheated conducting glass substrate. A simple three-electrode cell has been formed to study the electrochemical and electrochromic properties. The electrochemical parameters of the cell such as anodic peak current, anodic peak potential, threshold voltage, amount of H+ ions intercalated into and deintercalated out of the WO3 samples are calculated. The effect of film thickness on these parameters are studied. The extent of electrochromism and reversibility of the colouration/bleaching processes of various WO3 samples are described. The colouration efficiencies at 633 nm are calculated. The maximum colouration efficiency obtained for thicker film, is 56 cm(2)/C. The samples were found to be stable in 0.05N H2SO4 electrolyte up to 1 x 10(3) colour/bleach cycles. (C) 2000 Elsevier Science B.V. All rights reserved.