WO3 thin films have been obtained by evaporating high-purity WO3 powder at 5 x 10(-4) Pa on SiO2, Si3N4 and annealed at 500 degrees C for 6, 12, 24, 96, and 200 h. The film morphology, crystalline phase and chemical composition have been characterized through atomic force microscopy, grazing-angle x-ray diffraction, and x-ray photoelectron spectroscopy (XPS) techniques. The as-deposited film is amorphous. After annealing at 500 degrees C the films are well crystallized with preferential growth along the [200] plane of monoclinic WO3 (JCPDS 43-1035). The mean roughness and the surface area of the films are influenced by the nature of the substrate and the annealing time. The binding energies of the annealed films are close to those of WO3. High-resolution XPS valence-band spectra have shown a well-defined W metallic peak near the Fermi edge, confirming the occurrence of oxygen vacancies on the surface. The electrical response of the films in dry and NO2-rich atmospheres (0.7-7.2 ppm concentration in dry air) has been evaluated by de current mode.