Sol-gel-derived films of WO3 deposited on transparent conducting electrodes (TCEs) have been investigated by impedance spectroscopy in a gel electrolyte as a function of frequency from 200 kHz to 1 mHz at various applied voltages. These studies have revealed that electron injection at the TCE/WO3 interface occurring at high frequencies and charge transfer and diffusion controlled Li insertion into a WO3 film at low frequencies govern the colouration mechanism of WO3 films. The surface of WO3 films has been shown to be nearly homogeneous which has been reaffirmed by AFM studies. © 2002 Elsevier Science B.V. All rights reserved.