In this work, the intercalation/de-intercalation process of Li+ ions in the tungsten oxide matrix was investigated. The reaction mechanism involved was also investigated. The WO3 films, prepared by the polymeric precursor method, were deposited on a Pt covered quartz crystal using the dip-coating technique. The electrolyte was 0.1 mol L-1 LiClO4 in acetonitrile. The morphology and structure of the deposit was accomplished by scanning electron microscopy and X-ray diffraction, respectively. In the electrochemical quartz crystal nanobalance results, it was observed that the mass transport as well as the kinetic processes involved are facilitated in the films cycled at lower potential sweep-rates. The mass variation data as a function of the charge variations in the anodic and cathodic regions indicate the participation of solvent molecules (acetonitrile) during the Li+ ion intercalation/de-intercalation process. This was confirmed by the development of a model of the species flux as a function of the potential. (c) 2007 Elsevier B.V. All rights reserved.