Thin films of polycrystalline tungsten trioxide were manufactured using DC magnetron sputtering. Films of different thickness were deposited onto glass substrates coated with indium tin oxide (ITO). The crystallinity was confirmed by X-ray diffraction, and the grain size was found to be 30 nm. Li ions and electrons were intercalated into the sample using a three-electrode setup. The samples were submitted to optical characterization by spectrophotometry, in the visible and infrared ranges. The optical spectra were recorded at different intercalation states, and the absorption of the films was obtained. At low intercalation levels, a pronounced absorption peak was observed to be centered at a wavelength of 1.8 mum. Upon intercalation, the inserted electrons enter the conduction band, but due to a strong electron-phonon interaction, they are believed to form localized polarons. Calculations of optical absorption by large polaron theory were carried out and the position of the observed peak was in good agreement with the theory. A crossover from dielectric (low reflectance and clear phonon absorption bands) to metallic (high infrared reflectance) occurred at a Li intercalation level of around 0.05-0.15 Li/W. This may be due to overlap of the polaron states. (C) 2003 Elsevier B.V All rights reserved.