Lithium doped WO 3 electrochromic films have been deposited by thermal reactive evaporation of LiClO 4 and WO 3 ultra-pure powder mixture for different concentrations of the former from a Joule effect source, using an oxygen partial pressure of 5 × 10 −2 Pa. The substrate was a transparent conducting (20–25 Ω/) ITO film previously deposited on standard glass in the same run as the WO 3 deposition, and it was kept at 20Q °C of temperature during the WO 3 evaporation. Secondary ion mass spectrometry (SIMS) and X-ray photoelectron spectroscopy (XPS) analysis revealed intercalation of Li + and Cl − ions on the WO 3 layer. Chrono-amperometric measurements with simultaneous optical transmittance analysis were performed in order to evaluate the electrochromic colouring efficiency and also the role played by the Li + cations and the Cl − anions in the electrochromic structures.