Nanocrystalline WO3 films were produced by advanced reactive gas deposition onto alumina substrates. The as-deposited films had a tetragonal crystal structure and a mean grain size of around 6 nm, as found by X-ray diffraction and electron microscopy. Sintering at a temperature τS > 770 K yielded monoclinic films. We investigated the gas-sensing properties of films sintered up to 870 K. After an initial "activation" at τs = 750 K, the nanocrystalline WO3 films showed excellent gas-sensing properties, even at room temperature, on exposure to low concentrations of H2S in air. As little as 10 ppm of H2S made the conductance increase by a factor of about 103 within 10 min. The initial properties could be restored by heating the films to 530 K for 1 min.