With in situ micro-Raman measurements during the electrochemical reduction of WO3 thin films, the influence of the intercalated cation (H (+) /Li (+)) and an addition of water to the aprotic lithium electrolyte was investigated. The Raman spectra of lithium bronzes LixWO3 show two main results: (i) the intercalation of hydrogen can be clearly distinguished in situ from the intercalation of lithium with this technique and (ii) even with an addition of 500 ppm of water to the lithium electrolyte no hydrogen intercalation was observed. (C) 2002 Elsevier Science B.V. All rights reserved.