The sensing properties of the planar scandia-stabilized zirconia based mixed potential gas sensor coupled with hydrothermally obtained nanosized ITO as an electrode material have been investigated. The planar sensor has shown good sensitivity to low-level of CO concentrations (16- 500 ppm) in a lower oxygen ambient (4 vol %) in terms of the high magnitude of the signal and fast response (<5 s). The sensing properties of the planar sensor to CH4 and other oxidizing gases such as O-2 and NO2 have also been tested. The results indicate that the sensor could be used to potentially detect the reducing gases such as CO significantly rapidly compared to the oxidizing gases such as O-2 and NO2. A possible sensing mechanism of the planar sensor for rapidly detecting CO is also discussed. (C) 2005 The Electrochemical Society.