An oscillation phenomenon in ThO2-doped SnO2prepared by thick-film technology has been studied. The oscillation consists of two components at high sample temperature (210-230°C). When the sample temperature is kept at 210°C, a single-component oscillation with a sawtooth waveform starts to appear as CO-gas concentration increases. The activation energies of the surface reaction are 28.9 kcal/mol and 11.9 kcal/mol at the sample temperatures of 180-200°C and 210-230°C, respectively. From these experiments, it has been found that the coverage of oxygen on the surface plays an important role in establishing the oscillatory states.