Microstructure, electrochemical and optical properties of sol-gel deposited vanadium pentoxide (V2O5) coatings were investigated. The films were deposited by spin coating on SnO2:F/glass and quartz substrates from a polymeric solution of V2O5 derived from vanadium tri(isopropoxide)oxide. Process parameters were investigated for the deposition of V2O5 films exhibiting high lithium insertion capability. Investigations were conducted by cyclic voltammetry, impedance analysis, optical spectroscopy, scanning: electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. Structural investigations conducted by X-ray diffraction showed that the films fired at temperatures below 350 degrees C were amorphous, whereas those fired at higher temperatures were slightly crystalline. X-ray photoelectron spectroscopy (XPS) studies showed that the stoichiometry of the films was V2O5. The electrochemical behavior and structural changes were investigated in 1 M LiClO4/propylene carbonate solution. The stability of V2O5 films upon cycling and switching between oxidized and reduced states was investigated by cyclic voltammetry, and in-situ spectroelectrochemistry. Cyclic voltammetric measurements showed that V2O5 films exhibit good electrochemical cycling reversibility. In-situ optical measurement revealed that these films exhibit an electrochromic effect in the spectral range 300 nm < lambda < 2400 nm and change color between light yellow and light brown. The change in visible transmittance was 28% for 210 nm thick electrodes. XPS spectra indicate that V5+ is reduced to a lower valance state V4+ in a colored state with injected Li+. The bronze coloration is due to a simultaneous injection of electrons and Li+ ions into V2O5. The sol-gel deposited V2O5 films are useful for transparent counter electrodes in electrochromic devices. (C) 1997 Elsevier Science S.A.