Simultaneous removal of nitrogen monoxide and dinitrogen oxide with propene was investigated over Pd-H-ZSM-5 catalysts in comparison with reports of simultaneous removal of NO and N2O in the presence of O2 over Co-ZSM-5 and Fe-MFI. The N2O conversion was suppressed with the presence of NO, and the simultaneous removal rates were essentially the same as the NO conversion rates over Co-ZSM-5 and Fe-MFI. In contrast, the simultaneous removal in the presence of O2 was found to be accelerated over Pd-H-ZSM-5 and conventional Pd/SiO2 compared to the sum of the corresponding individual removal rates of NO and N2O. An intense FT-IR peak of NOｱ0 ligated to the Pd cation inside the ZSM-5 cage was observed at 1868 cm-1 in the promoted simultaneous removal reactions. Under the conditions for the individual removal of NO or N2O and the unpromoted simultaneous removal of NO + N2O, only a weak peak at 1842 cm-1 was observed. The oxygen atom that dissociates from N2O should be the key to promoted catalysis in the coexistence of NO and N2O in order to maintain the Pd cation site clear. The oxygen controls the concentration of surface carbonaceous species to effectively decompose NO, and not to inhibit the NO activation.