Tungsten oxide is shown to be a very promising material for the fabrication of highly selective ammonia sensors. Films of WO3 were deposited onto a silicon substrate by means of the drop-coating method. Then, the films were annealed in dry air at two different temperatures (300 and 400 degrees C). X-ray photoelectron spectroscopy was used to investigate the composition of the films. Tungsten appeared both in WO2 and WO3 oxidation states, but the second state was clearly dominant. Scanning electron microscopy results showed that the oxide was amorphous or nanocrystalline. The WO3-based devices were sensitive to ammonia vapors when operated between 250 and 350 degrees C. The optimal operating temperature for the highest sensitivity to ammonia was 300 degrees C. Furthermore, when the devices were operated at 300 degrees C, their sensitivity to other reducing species such as ethanol, methane, toluene, and water vapor was significantly lower, and this resulted in a high selectivity to ammonia. A model for the sensing mechanisms of the fabricated sensors is proposed.