Abstract: A Si chip (7 x 7 x 0.4 mm) was constructed in two parts; one is the gas-sensitive area and the other a temperature control area. The former consisted of two noble metal films (Au and Pd) deposited on titanium oxide. The latter was made up of a Ni-chrome heater and Ni thermo-sensor having a positive thermal coefficient. The heater, thermo-sensor and bonding pads were prepared by means of RF sputtering and photolithography on a Si substrate by the method of Toda, (Bunseki Kagaku, 1990, 39, 611). On the substrate a Ti film was deposited and its surface was oxidized at 500°C for 70 min in a stream of O2. Au and Pd films were deposited separately on the titanium oxide by means of evaporation. Test gases of various concentrations were prepared and allowed to flow over the sensor in a chamber at a controlled humidity of 65% (details given). For H2, the Pd-side unit exhibits a response related to the gas concentration while the Au-side shows no response. For phosphine, the Au-side exhibits an extremely high sensitivity while the Pd-side shows only a small response, even at 200 ppm.