The limiting current characteristics with regard to O-2 and CO were studied in oxidizing (O-2-CO2-N-2) and reducing(CO-CO2-N-2) atmospheres. The experimental I-V characteristics were compared with a simulation and discussed considering the electrochemical oxidation and decomposition between CO and CO2. The decomposition of CO2 was one of the main O-2 sources of oxygen pumping in reducing atmospheres. The limiting current with regard to CO in reducing atmospheres showed a sloped limiting current regime at 700-725 degrees C, while that with regard to O-2 in oxidizing atmospheres did not. The sloped limiting current of CO resulted from electrode reactions which were different from those in oxidizing atmospheres, i.e. the CO2 decomposition for the O-2 pumping cell and the CO oxidation for the O-2 consuming cell.