Electrodeposition of amorphous electrochromic tungsten trioxide (WO3) was studied, Good quality films were deposited cathodically onto tin-doped indium oxide (ITO) from a water/isopropanol solution containing dissolved tungsten(VI) species. Three reducible species are present in this solution : unbound H2O2, polytungstate ions, and peroxytungstate ions, The effects of the H2O2 content, which is related to the age of the deposition solution, and of mass transport on the deposition current and deposition efficiency were determined and used to elucidate the mechanism of electrodeposition. Reduction of unbound H2O2 and polytungstate did not result in film growth. This part of the current was under mixed diffusion-kinetic control. The remainder of the current was due to reduction of peroxy-tungstate and was kinetically limited. This process led to WO3 deposition. The current efficiency, defined as the number of electrons consumed by reduction of peroxy-tungstate per W atom deposited, was about 1.6, in good agreement with the proposed deposition reaction.