An oscillation has been found in the resistivity of gas sensing devices of sintered SnO2mixed with Pd, Rh, and MgO exposed to H2gas in air. This phenomenon depends on the H2gas concentration, the working device temperature, and the voltage applied to the device. Its waveforms consist of two structures and are conspicuously noticeable at 92 °C device temperature and 40 V voltage applied across the device.