Substoichiometric tungsten oxide films (WO(3-y), 0.49 >= y >= 0.15) were prepared by non-reactive thermal evaporation Of WO(3) powder in vacuum. The thin film composition, structure and optical properties were investigated with the purpose to establish their dependence on the deposition conditions and to prove a possible correlation between electrochromic and gasochromic colouration. An analogy in the dependencies of the maximum achievable optical density on the thin film oxygen content for gasochromically and electrochromically coloured films was observed. In-situ performed XPS measurements suggested that the main mechanism of gasochromic colouration is charge transfer between W(6+) and W(5+) states, i.e., similar to the electrochromic effect. (C) 2008 Elsevier B.V. All rights reserved.