A planar NO2 sensor was fabricated by using an yttria-stabilized zirconia plate and NiO-based sensing electrode (SE). The improvement in sensing characteristic of mixed-potential-type planar NO2 sensor was examined by the addition of various metal oxides to NiO-SE. Among the metal-oxide additives tested, WO3 was found to give a significant enhancement in NO2 sensitivity. In addition, this enhancement was maximum when the initial WO3 content in the NiO-based paste before sintering was 10 wt %. The sensitivity (Delta electromotive force) to 50 ppm NO2 was as high as about 70 mV for the sensor using the 10 wt % WO3-added NiO-SE at 800 degrees C under the wet condition (containing 5 vol % H2O). The sensitivity of this sensor was still 8 mV even down to 1 ppm NO2 at 800 degrees C. The NO2 sensitivity was found to be invariant to different concentrations of H2O and CO2 in the examined range of 5-15 and 5-20 vol %, respectively. It is speculated that the modification of morphology of the NiO-SE layer by the addition of WO3 is the main contributing factor for the improved NO2 sensitivity of the sensor. (c) 2007 The Electrochemical Society.