We have investigated the electrochromic properties of amorphous granular tungsten oxide (WO3+delta) thin films with over-stoichiometric oxygen content (delta), using LiClO4 with propylene carbonate as an electrolyte. Different optical and electrochromic characteristics are observed with increasing delta. All the devices are electrochemically stable for more than 5000 color/bleach cycles without apparent degradation, and they have a faster response to coloration than to bleaching. WO3+delta films with an optimized delta value show an optical modulation of 86% at a wavelength of 630 nm and the highest coloration efficiency ever reported of similar to 213 cm(2)/C. The delta-dependent coloration mechanism is discussed using the site saturation model. It is proposed that WO3+delta films with the optimal delta value have favorable thickness and stoichiometry for the generation of Li+W+5 states. (C) 2012 Elsevier B.V. All rights reserved.