Electrochemical sensors based on yttria-stabilized zirconia (YSZ) with WO3 as a sensing electrode were fabricated using either Pt or Au electrodes. The sensors were studied in the temperature range 500-700°C in the presence of different concentrations (300-1000 ppm) of NO2 and CO in air. The response to NO2 was very stable with fast response time (within 20 s). The best sensitivity was observed at 600 and 650°C using Pt and Au electrodes, respectively. At all temperatures investigated a cross sensitivity to CO gas was also noticed. The response to CO was decreased using Au electrodes. The role played by WO3 as a sensing electrode was investigated. © 2003 The Electrochemical Society. All rights reserved.