Devices based on single-crystalline In2O3 nanowires were used to detect ammonia gas via electrical measurements at room temperature. Interesting phenomena have been observed, as the direction of the conductance modulation upon NH3 exposure depends on both the initial state of the nanowire surface and the nanowire doping concentration. For nanowires with surfaces cleaned via UV illumination in a vacuum, the conductance has been found to increase for lightly doped nanowires and decrease for heavily doped nanowires upon exposure. In contrast, for nanowires residing in ambient atmosphere, i.e., under practical conditions, the conductance has been consistently observed to increase upon NH3 exposure, regardless of the doping concentration. This is explained by considering electron transfer between In2O3 nanowires and NH3. Our work clearly demonstrates the potential of using In2O3 nanowires as NH3 sensors under practical conditions.