Tungsten trioxide (WO,) thin films were prepared by spray pyrolysis. The optical and electrical properties of intensively colored films were studied. It was found that the near-infrared reflectivity increases in the colored state. This behavior is typical of degenerate semiconducting materials. However, the variations of the electrical conductivity with the temperature for these films, indicates semiconducting behavior, while the carrier concentration (n), measured by the Hall effect, is typical of degenerate material. The optical properties can be easily explained by the Drude theory, which is related to the bulk properties of the samples. It is shown that the value of n is also related to the bulk proper-ties of the films, while the conductivity is controlled by the grain boundary effect. The results are interpreted in terms of a modified version of Seto's model, taking into account thermionic emission and tunneling of carriers through the potential barriers at the grain boundaries.