Semiconductor gas sensors based on nanocrystallline WO3 films were produced by two different methods. Advanced reactive gas evaporation was used in both cases either for a direct deposition of films (deposited films) or to produce ultra fine WO3 powder which was used for screen printing of thick films. The deposited films sintered at 480°C and the screen-printed films sintered at 500°C displayed a mixture of monoclinic and tetragonal phases and had a mean grain size of approximately 10 and 45 nm, respectively. We studied the influence of the sintering temperature Ts of the films on their gas sensitivity. Unique and excellent sensing properties were found upon exposure to low concentrations of H2S in air at room temperature for both deposited and screen-printed films sintered at Ts = 480°C and at Ts = 500°C, respectively. © 2001 Elsevier Science B.V.