A semiconductor gas sensor using SnO2 was loaded with acidic or basic oxides (5 wt.%) to investigate ethanol-gas sensing properties and related catalytic properties. The sensitivity to ethanol gas at 300 degrees C increased tremendously with an addition of a basic oxide (e.g., La2O3), while it hardly changed with that of an acidic oxide (WO,). It turned out that the addition of the basic metal oxide to SnO2 brought about enhancement of catalytic activity not only for the dehydrogenation of ethanol gas to CH3CHO but also for the consecutive oxidation of CH3CHO to CO2. On the other hand, the acidic metal oxide enhanced only the dehydration reaction, showing even an adverse effect on the consecutive oxidation. Based on these results, it was concluded that the enhancement of the catalytic oxidation activity to an appropriate level could be a reason for the high sensitivity to ethanol gas for the sensors loaded with basic oxides, particularly one loaded with La2O3.