The influence of reduction and reoxidation reactions on NOx decomposition properties of an electrochemical cell with a catalytic layer (NiO/yttria stabilized zirconia) has been studied. To clarify the correlation between the NOx decomposition properties and the redox reactions of NiO in the catalytic layer, we carried out the measurements of reoxidation time dependence of electric current in the temperature range of 350-600 S C. The electrochemical cell with the catalytic layer was shown for the highly effective NOx decomposition properties in the presence of excess oxygen. Even if the reoxidation time increases, the current efficiency for NOx decomposition was not deteriorated at less than 400 S C. However, the current efficiency has been decreased according to an increase in temperature and/or reoxidation time at over 450 S C. The deterioration of current efficiency occurs with the volume expansion of the catalyst layer by the reoxidation of NiO. (c) 2007 The Electrochemical Society.