Amorphous and crystalline WO3 films show electrochromic properties by means of double injection of cations and electrons. Absorption by small polaron transitions has been hypothesised to explain the coloration mechanism in amorphous WO3 films and the most reliable model was found by Zhang, who proposed that a WO3-x bleached virgin film (x < 0.4) consists mainly of W6+ and W4+, whereas in the coloured state W5+ is also present. In this work, amorphous WO3 films have been deposited by a thermal evaporation technique on glass and c-Si substrates. Electrical, optical and IR properties of these samples have been studied. A hopping mechanism with mobility activation has been hypothesised to justify the discrepancies between electrical and optical properties. Furthermore, our careful analysis of the typical IR absorption peaks in the range 500-1000 cm(-1) confirmed the Zhang theory on the coloration mechanism. (C) 2003 Elsevier Science B.V. All rights reserved.