We propose a new model for the chromic mechanism in amorphous tungsten oxide films (WO3-y. nH(2)O). This model not only explains a variety of seemingly conflicting experimental results reported in the literature that cannot be explained by existing models, it also has practical implications with respect to improving the coloring efficiency and durability of electrochromic devices. According to this model, a typical as-deposited tungsten oxide film has tungsten mainly in W6+ and W4+ states and can be represented as W1-y6+Wy4+O3-y. nH(2)O. The proposed chromic mechanism is based on the small polaron transition between the charge-induced W5+ state and the original W4+ state instead of the W5+ and W6+ states as suggested in previous models. The correlation between the electrochromic and photochromic behavior in amorphous tungsten oxide films is also discussed.