Tungsten trioxide (WO3) is a well-known electrochemical material due to its ability to change its optical properties under the lithium intercalation/deintercalation process, and it is considered one of the best materials for practical electrochromic devices. In this paper we present the electrochemical and optical properties of lithiated and unlithiated thin films of tungsten trioxide obtained by the sol-gel process. The knowledge of transport kinetics parameters in thin solid films is of great importance to understand the behavior of these films during insertion/extraction processes. The response time of the electrochemical process of the films is determined by the rate of charge transfer and mass transport. In order to study the solid state diffusion of lithium into WO3 films, two electroanalytical techniques have been used, i.e. galvanostatic intermittent titration technique (GITT) and electrochemical impedance spectroscopy (EIS). The values of the diffusion coefficient for the WO3 and lithiated WO3 films were determined. (C) 2003 Elsevier B.V. All rights reserved.