The non-Nernstian behavior or zirconia-based electrochemical cells with various metal oxide electrodes for 0-500 ppm C1-C4 hydrocarbons, hydrogen, and carbon monoxide was studied in the presence of 10% oxygen at 600 degreesC. The cell using an In2O3 electrode exhibited a largely negative electromotive force (emf) for propene but gave moderately negative emfs for hydrogen and carbon monoxide. The addition of 0.1 wt % MnO2 to the In2O3 electrode significantly improved the selectivity of the parent electrode for propene over hydrogen and carbon monoxide. The negative emf of the cell using the MnO2-containing In2O3 electrode for the hydrocarbons became enhanced as the carbon number of the hydrocarbon increased, the C-E linkage was unsaturated, and the chain structure was branched. This enhanced effect, as well as the addition effect of MnO2, was discussed on the basis of the measurements of the polarization curves of the MnO2-containing In2O3 electrode for the hydrocarbons and oxygen and the nonelectrochemical oxidation of the hydrocarbons by oxygen. In addition, the MnO2-containing In2O3 electrode for the hydrocarbons was compared with an An electrode with respect to sensitivity to the hydrocarbons and reproducibility of the sensing properties. (C) 2000 The Electrochemical Society. S0013-4651(00)07-09 1-9. All rights reserved.