The non- Nernstian behavior of zirconia- based electrochemical NOx and CO sensors with various semiconducting oxides as sensing electrodes was extensively studied in the temperature range 450-700degreesC. Both pellets and tape- cast layers of yttria-stabilized zirconia were used for the fabrication of the sensors. It was found that the gas- sensing mechanism of the sensors cannot be explained always by the mixed potential theory. When semiconducting oxides are used as sensing electrodes, the semiconductor sensing mechanism and the adsorption-desorption behavior of the different oxides dominate the sensor response if both metal and oxide are exposed to the same environment. (C) 2004 The Electrochemical Society.