There is an increasing demand for semiconducting gas sensors for several monitoring applications that have sensitivity, selectivity and reliability on a long-term scale. In this paper, we have described the preparation of SnO2-Au thin film sensors by the RGTO technique; these sensors have proved to be capable of sensitive and selective detection of CO. The TEM and AES analysis showed that 6 months of sensor aging at 400 degrees C did not produce any valuable rearrangement of gold atoms onto a tin dioxide surface, Most of the sensor resistance variation was observed during the first 20 days, and a limited drift was observed in the remaining period.