Ammonia gas sensing Properties of a single-layer In2O3 doped with 5 mole percent (m/o MgO [In2O3-MgO (5 m/o)] sensor and some double-layer sensors with a catalyst layer on it have been investigated. The single-layer sensor with electrodes at the innermost region exhibited low NH3 sensitivity due to interference from NO(x) produced as a result of complete oxidation of NH3, especially at temperatures higher than 530-degrees-C. A slight enhancement sensitivity was achieved by coating the In2O3-MgO (5 m/o) sensing layer with a catalyst layer. A more remarkable sensitivity enhancement was realized by changing the electrode position of double-layer sensors from the innermost to the interface between the sensing and the catalyst layer, especially when TiO2 loaded with 0.5 weight percent Ir was employed as a catalyst layer. Based on the catalytic activity of the sensing and the catalyst materials, possible NH3 sensing mechanism of sensors is discussed.