To elucidate the reactions involved in the three-way catalyst under stoichiometric conditions, the reactivity of CO + NO has been studied by continuous flow and surface reactions over Pd supported on Ce0.75Zr0.25O2 (CZO) and Al2O3 model catalysts with different Pd particles sizes and oxidation states. Pd/CZO showed higher activity with complete NO removal at 125 °C than Pd/Al2O3 at 300 °C As a primary by-product, N2O was produced on both catalysts prior to reaching complete CO conversion in a 1:1 NO/CO feed ratio. NO inhibition through N2O formation was intensified when 2:1 NO/CO feed ratio was used, where the complete CO conversion can’t be achieved during the course of the reaction. When 1:1 NO/CO feed ratio was used over Pd/Al2O3, a concave feature in the CO light-off curve was observed, where the CO light-off curve showed a downward inflections after complete NO conversion had been reached, and then started to increase again with further increase in temperature. The inflection of CO conversion coincided with the maximum yield of N2O vs. reaction temperature. It was also found that the CO + NO reactivity was affected by the Pd oxidation state and particles size. in situ DRIFT experiments showed that the formation of NCO and NCO-derived N2O intermediate species is closely related to the inflection of CO conversion. A pathway of N2O formation via the NCO intermediate species has been proposed to be responsible for the fallback in CO conversion when the reaction temperature increased, as both interactions of “NCOad + Nad” and “NCOad + NOad," which produce N2 and N2O, respectively, consume NO but generate CO.