In an effort to develop a semiconductor type NH₃ sensor, the NH₃ sensing properties of WO₃ semiconductors loaded with colloidal noble metals were investigated. Among the various noble metals tested, Au was proven to be the most excellent promoter for the WO₃-based NH₃ sensor, whereas Pt gave a very high NH₃ sensitivity at 200°C, but the response was too slow. Thus Au-WO₃ element could respond to NH₃ in air over a wide NH₃ concentration range of 5 ppb-50 ppm at 450°C, showing excellent characteristics in both NH₃ sensitivity and response rate. An optimum Au loading amount was shown to be 0.8 wt % with respect to the NH₃ sensitivity. The electrical resistance of Au-WO₃ element in air increased with an increase in Au loading. From the measure-ments of Seebeck coefficients it was revealed that the Au particles loaded interact with WO₃ and take the conduction electrons away when exposed to air, while those electrons are given back to WO₃ on exposure to NH₃-containing atmospheres. It is concluded that such an electronic interac-tion between Au and WO₃, susceptible to a change in atmosphere, is responsible for the prominent promoter action of Au.