The catalytic activity for CO, hydrocarbon and NO removal on Al2O3 and CeO2 based oxides supported Pd catalysts were studied under switching of aging condition between air and N-2 atmosphere. For CeO2-containing Pd catalysts, the deterioration of catalytic activities by aging in N-2 was improved by oxidative treatment. Based on results of XPS and FT-IR measurements, it was presumed that the catalytic activity of Pd catalysts was strongly affected by the adsorbed form of CO on Pd, which occurs owing to a change of the chemical state of Pd.