Hitherto unexplored irreversible changes during initial coloration/bleaching cycles for sol-gel-derived tungsten oxide (WO3) films have been investigated using cyclic voltammetric and spectrophotometric techniques. Non-ideal features appearing in the initial five anodic (deintercalation) cycles in the voltammogram with simultaneous decreased optical transmission of the bleached films have been explained in terms of possible stoichiometric variations affecting the coloration efficiency (CE) of the films and the associated mechanisms. Electrochromic stability attained thereafter manifests in retraceable voltammograms and almost invariant value of the CE. (C) 2005 Elsevier B.V. All rights reserved.