Electrochemical planar sensors based on yttria-stabilized zirconia (YSZ) with semiconducting oxides (WO3 and LaFeO3) and mixed conductors (La0.8Sr0.2FeO3) as sensing electrodes were investigated. The electromotive force (EMF) of the sensors was measured at fixed temperature (450-700 degreesC) and different concentrations of NO2 and CO in air in the range 20-1000 ppm. The sensors were wholly exposed to the same gas atmosphere, alternatively air and different gases in air, without using reference air. Fast and stable response was measured for all the different sensors. The EMF of p- and n-type semiconducting-based sensors was measured in opposite directions upon the same gas exposure. These results obtained for the response of planar sensors with different semiconductor sensing electrodes showed that the sensing mechanism cannot be explained by mixed potential theory. The response of these sensors is better described by a differential electrode equilibria sensing mechanism. (C) 2004 Elsevier B.V. All rights reserved.