Electrochromic tungsten trioxide thin films were prepared by chemical vapor deposition. The source material was tungsten carbonyl. Amorphous tungsten trioxide thin films were obtained at a substrate temperature of 300-degrees-C. Reduction and oxidation of the films in a 0.3M LiClO4 propylene carbonate solution resulted in desirable changes in optical absorption. Coulometry indicated that the coloration efficiency was 230 cm2 . C-1.