Desorption behavior of gases from the NH3-preadsorbed catalyst and sensing materials used for double-layer semiconductor NH3 sensors has been investigated under different adsorption and desorption conditions. The 0.5 Ir/TiO2 catalyst material was found to exhibit mild activity for NH, oxidation and high activity for reducing NO to N-2 in an atmosphere containing NH3 and O-2. This nature was considered to be effective for reducing the interference from NOx at the vicinity of the interface between the catalyst layer and the In2O3 sensing layer doped with 5 mol% MgO. The reduction of the interference led to high NH, sensitivity and a normal response to give a resistance decrease upon exposure to NH3 even at higher temperatures. The abnormal response of a double-layer sensor with a 0.5 Pt/TiO2 catalyst layer to give a resistance increase upon exposure to NH3 especially at higher temperatures was confirmed to arise from its high NH, oxidation activity and low NO reduction activity.