Mixed phase tungsten trioxide-vanadium pentoxide films with compositions ranging from 0 to 10 mol% vanadium pentoxide were deposited by the sol-gel for electrochromic electrodes. Colorless two phase films with properties equivalent or superior to tungsten trioxide were formed. The coating solutions were prepared by reacting a peroxytungstic ester with vanadium triisopropoxide in ethanol. Amorphous thin films ranging from 260-292 nm thick were deposited by spin coating with a spin rate of 1200 rpm. The films were heat treated at 100 degrees C in air for 1 h. Film characterization was made by cyclic voltammetry (CV), optical and Fourier infrared (FT-LR) spectroscopy, scanning electron microscopy SEM, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). XRD determined the films were amorphous in structure. The XPS examination for the composite films showed the characteristic peaks of V2O5 and WO3, which indicated the films an composed of a two phase mixture. Electrochemical lithium insertion into (1 - x)WO3-(x)V2O5 films was investigated. An in situ three electrode cell with a 0.5 M LiClO4/propylene carbonate electrolyte was used. Cyclic voltammetric measurements showed that tungsten trioxide-vanadium pentoxide films exhibited good reversibility of Li+ and e(-) insertion/extraction process in the (1 - x)WO3-(x)V2O5 oxide with constant charge capacity. The best electrochromic performance was observed for (0.965)WO3-(0.035) V2O5 film. The electrochromic color was a neutral brownish blue for this two phase oxide. (C) 1999 Elsevier Science S.A. All rights reserved.