Tin dioxide based sensors with different additives were constructed and tested in air environment containing carbon monoxide. Conductance oscillations were observed in samples containing palladium but not in those without. Oscillations occurred at temperatures ranging from 150℃ to 320℃. Within this temperature region the range of CO concentrations at which oscillations appeared became higher as the test temperature increased. The lowest CO concentration at which oscillations were observed was 200 ppm and the highest 10000 ppm.By comparing sensor responses obtained in synthetic and ambient air it was concluded that water vapour has a major influence on oscillations and increases the frequency. The ranges of CO concentrations in which oscillations occurred at different temperatures, however, remained roughly the same in both environments. It was also noticed that processing conditions had an influence on the oscillatory response characteristics of the sensors.