Intercalation of lithium lions in amorphous tungsten oxide films and in monoclinic polycrystalline tungsten oxide films has been studied using electrochemical analyses and Raman spectroscopy. We present direct evidence for W5+ 5d-electron localization in amorphous tungsten oxide films upon lithium ion and electron intercalation by comparing the Raman spectra of amorphous and crystalline Jungsten oxide films.' We observe that when lithium ions and electrons are inserted into the crystalline tungsten oxide films, the monoclinic structure of the material proceeds to. progressively increased symmetry. On the other hand, when lithium ions and electrons are inserted into amorphous tungsten oxide films, the inserted electrons are localized in W5+ sites and polarize their surrounding lattice to form small polarons: The extra Raman peaks due to the W5+-O single bonds and W5+=O double bonds appear at 330 and 450 cm(-1), respectively. (C) 2003 Elsevier Science B.V. All rights reserved.