The phenomenon of electrochromism in tungsten trioxide (WO3) thin films has recently attained considerable interest due to their enormous applications in inorganic thin film electrochromic devices. We have investigated the compositional, optical, and electrochromic properties of the WO3 thin films grown at different substrate temperatures by the thermal evaporation of WO3 powder. The thin films were characterized using X-ray diffraction (XRD). X-ray photo-emission spectroscopy (XPS), and electrochemical techniques. The XPS analysis suggested that the oxygen to tungsten (O/W) ratio decreases, i.e., the oxygen deficiency increases, on increasing the substrate temperature up to 500 degrees C. The electrochemical analysis provided a comparative study of the coloration efficiency (CE) of the WO3 thin films intercalated with three different ions viz. H+, Na+, and K+. The effect of the variation of the substrate temperature on the CE and the switching time have also been investigated for the WO3 thin films intercalated with H+ ions; the thin films deposited at RT and intercalated with H+ ions are found to possess adequate electrochromic properties viz. CE and switching time from device point of view. (C) 2010 Elsevier B.V. All rights reserved.