Amorphous tungsten oxide films were made by sputtering onto glass substrates that were coated with conductive tin doped indium oxide (ITO). The films were deposited at different O-2/Ar gas flow ratios and different substoichiometric compositions was determined by Elastic recoil detection analysis (ERDA). Substoichiometric as-deposited tungsten oxide is transparent above a particular oxygen content and is blue below that content. This indicates that there are at least two kinds of defects in the substoichiometric films. The oxygen vacancies may be coupled to W5 + SiteS, giving rise to strong absorption, or to (W-W)(10 +) complexes in the transparent films. Lithium ions were electrochemically intercalated at several charge levels. At each level the transmittance and reflectance were measured in the wavelength range between 0.3 and 2.5 mum. We show that as-deposited blue films and intercalated transparent films display similarly shaped optical absorption bands. Electrochromic devices were made by laminating the tungsten oxide films with sputtered Ni-V oxide deposited on ITO-coated plastic substrates. The durability under electrochemical cycling was best for the case of very substoichiometric WO2.63 films. (C) 2003 Elsevier B.V. All rights reserved.