The mechanism of the NO decomposition in the presence of excess O-2, H2O and CO2 using a ceria-based oxide ionic conductor with a Pd electrode has been studied by solid electrolyte potentiometry (SEP) method. A single compartment cell for measuring P-O2, PtYSZPt, are attached to another single compartment cell for decomposing NO, PdCe0.8Sm0.2O1.9Pd, where their working electrodes come into contact with each other. Experiments were carried out with a mixture of 2611 ppm NO, 0.1-4% O-2, 0-5% H2O and 0-5% CO2 in argon at 800 degrees C. A P-O2 measurement in the neighborhood of the Pd electrode during NO decomposition confirms that the NO decomposition is initiated after the electrochemical oxygen pump is completed. A comparison of the Pd electrodes heated at 850 degrees C with that heated at 1300 degrees C shows that the poorer the oxygen pumping property, the better the NO decomposition. On the basis of these results, the mechanism of the NO decomposition is discussed in detail. In addition, the SEP method also gives information about the influences of the coexistent O-2, H2O and CO2 on the NO decomposition.