The byproduct formation in environmental catalysis is strongly influenced by the chemical state and coordination of catalysts. Herein, two Pd/CeO 2 catalysts (PdCe-350 and PdCe-800) with varying oxygen vacancies (O v ) and coordination numbers (CN) of Pd were prepared to investigate the mechanism of N 2 O and NH 3 formation during NO reduction by CO. PdCe-350 exhibits a higher density of O v and Pd sites with higher CN, leading to an enhanced metal-support interaction by electron transformation from the support to Pd. Consequently, PdCe-350 displayed increased levels of byproduct formation. In situ spectroscopies under dry and wet conditions revealed that at low temperatures, the N 2 O formation strongly correlated with the O v density through the decomposition of chelating nitro species on PdCe-350. Conversely, at high temperatures, it was linked to the reactivity of Pd species, primarily facilitated by monodentate nitrates on PdCe-800. In terms of NH 3 formation, its occurrence was closely associated with the activation of H 2 O and C 3 H 6 , since a water-gas shift or hydrocarbon reforming could provide hydrogen. Both bridging and monodentate nitrates showed activity in NH 3 formation, while hyponitrites were identified as key intermediates for both catalysts. The insights provide a fundamental understanding of the intricate relationship among the local coordination of Pd, surface O v , and byproduct distribution.