Ultra-fine powder of tungsten oxide was made by evaporation of tungsten metal by an electric arc discharge in a reactive atmosphere. The obtained WO3 powder displayed a mixture of monoclinic and tetragonal phases and had a mean grain size of 40nm. We investigated the gas sensing properties of nanocrystalline thick-films made from the powder and studied the influence of the sintering temperature τs of the films on the gas sensitivity. The WO3 films showed excellent sensing properties upon exposure to low concentrations of H2S in air at room temperature. At exposure to 10ppm of H2S, the conductance increased by a factor of about 104 for films prepared at τs=300°C. A further rise of τs for the films made the enhancement of their room temperature conductance upon H2S exposure go down, and the H2S sensitivity disappeared after treatment at τs600°C. This loss of sensitivity occurred concomitantly with the disappearance of the tetragonal phase, which may point at a specific crystal structure being responsible for the unique gas sensing properties of nanocrystalline WO3. © 2001 Elsevier Science B.V.