A SnO₂ sensor has been tested under hydrogen or methane pollutant gases with varying the oxygen concentration in the supporting gas (nitrogen). The oxidation reactivity of the SnO₂ material has been evaluated by the use of a hygrometer, which in the dynamic flow system beyond the sensor cell, measures the water evolved when the H₂ (or CH₄)/SnO₂ interaction takes place. The results, expressed by the electrical response as a function of oxygen and hydrogen (or methane) concentrations, give information about the detection mechanism. For hydrogen gas detection, two different processes seem to occur, an adsorption-reaction mechanism when the oxygen concentration is high in comparison with the hydrogen one and an electrochemical response in the reverse case. For methane gas, the sensor response mechanism is probably independent over the range of oxygen and methane concentrations investigated.