Thin films were obtained by r.f. reactive sputtering from a Ti0.1–W0.9 target followed by heating in dry air. Their morphological and structural characteristics were studied by means of electron-microscopy techniques. Annealing at 600°C of a sample leads to a WO3 polycrystalline thin-film. A solution of Ti ions in the WO3 lattice causes the film to have higher surface-to-volume ratio and thereby higher gas-sensitivity with respect to pure WO3 sputtered thin films. This layer is highly sensitive to NO2 as concentrations lower than 1 ppm of NO2 are detected. Annealing the film at 800°C results in a TiO2-anatase nanostructured layer which is capable to detect few ppm of NO2 in dry air within the temperature range of 350–800°C.