Solid state gas sensors based on thin films of SnO2 can be very sensitive to small amounts of hazardous gases like hydrocarbons and carbonmonoxide. There are two models to explain the observed change in conductance, when the sensors are exposed to the gases: (1) Adsorption of the gas molecule at the surface and electron transfer from the gas molecule to the SnO2 or (2) adsorption of the gas followed by a reaction of the gas with previously adsorbed oxygen. The conductivity change is caused by the released electron which was captured by the oxygen during its adsorption. Experiments were carried out which support the second model. In order to get a stable gas sensor it is necessary to keep the surface of the SnO2 thin film in such a condition that the oxygen adsorption/desorption process can operate for a long time.