A series of undoped SnO2 films was grown by metallorganic chemical vapor deposition (MOCVD) from an Sn(O(t)Bu)(4) precursor. The characterization of their CO- and H2O-gas sensing properties is reported. The films were very sensitive to low levels of CO at elevated temperatures (200 to 400 degrees C), although a significant cross sensitivity to relative humidity was found. Response and recovery times were very short in comparison with a Pt-doped thick film SnO2 pellet. The optimum working temperature was found to be 300 to 350 degrees C, where linear responses to CO concentration and to relative humidity were seen once drift had been taken into account. All MOCVD films tested showed superior responses to a sputtered film.