The effect of thermal annealing on the electrochromic properties of the tungsten oxide (WO3-x) nanowires deposited on a transparent conducting substrate by vapor evaporation was investigated. The X-ray diffraction (XRD) indicated that the structures of the nanowries annealed below 500 degrees C had no significant change. The X-ray photoelectron spectroscopy (XPS) analysis suggested that the O/W ratio and the amount of W6+ ions in the annealed nanowire films could be increased as increasing annealing temperature. Increased annealing temperature could promote the coloration efficiency and contrast of the nanowire films-, however, it could also affect the switching speed of the nanowire films. (C) 2007 Elsevier B.V. All rights reserved.