SnO2 thin film gas sensors were prepared by r.f. magnetron sputtering. It was shown that the gas-sensitive properties of SnO2 thin films are affected substantially by the conditions of preparation which determine the oxygen content in the films. Oxygen-deficient films were fabricated in two manners: (1) deposition in Ar atmosphere followed by annealing in an oxygen atmosphere, and (2) deposition in Ar/O-2 mixture. Both methods facilitated the fabrication of polycrystalline films with a high sensitivity to reducing gases. However, the long-term instability of the film conductance caused by oxygen diffusion at advanced operating temperatures did not appropriately meet the sensor requirements. Doping of the films with copper atoms was shown to be a useful method of stabilizing the electrical characteristics. The mechanism of stabilization is proposed.