We report the gas-sensing properties of ion-beam sputter deposited MoO3 thin-films. The change in the DC conductivity was measured in dry N-2 with 10% O-2 in the presence of up to 490 ppm of NH3, NO, NO2, C3H6, CO and H-2. At similar to-440 degreesC the film was found to be very sensitive to NH3, with 490 ppm increasing the conductivity by approximately a factor of 70. This was approximately 17 times greater than the response to the other gases. The NH3 response was strongly affected by the accompanying levels of O-2, NO2 and H2O. For example, changing the accompanying O-2 levels from 1% to 20% decreased the NH3 response by approximately a factor of 20. Similarly, the presence of 100 ppm NO2 (in 10% O-2) decreased the NH3 response by approximately a factor of three, and 1% water vapor decreased it by more than a factor of two. The NH3 response, however, was relatively unaffected by 100 ppm of accompanying NO, C3H6, CO or H-2. XPS measurements show that the increased conductivity in the presence of NH3 was also accompanied by a partial reduction of the surface MoO3. We observed an increase in the resistance of the films after extended time at elevated temperatures. (C) 2003 Elsevier Science B.V. All rights reserved.