Fine-grained crystalline or amorphous thin films are normally produced by atomistic deposition of species under conditions giving a low adaton mobility. Here we consider an alternative high-temperature route employing advanced reactive gas deposition for structure engineering of WO3 nanoparticles in making nanocrystalline films, especially, for gas sensing applications. For instance, structure engineering may be necessary in the case of very fine-grained n-type semiconductor oxide films in order to increase the electrical conductivity of the films. The crystal structure of WO3 together with its different phases makes possible structure engineering of its nanoparticles by high-temperature processes such as the reactive gas deposition. Produced nanocrystalline WO3 films in metastable tetragonal crystal structure were found to be very sensitive to H2S in air already at room temperature. © 2003 Elsevier Ltd. All rights reserved.