In situ X-ray absorption spectroscopy at the Ce Lm edge and Pd K edge was used to characterize the oxidation state and structural parameters of CeOx/Al2O3 and Pd/CeOx/Al2O3 catalysts during the reduction of NO by CO. The samples were exposed to oxidizing (5% NO in He), reducing (5% CO in He), and stoichiometric (5% NO/5% CO in He) gaseous environments. The Ce LIII edge structure indicated that one-third of the cerium readily changed oxidation state between 3+ and 4+ upon exposure to various mixtures of NO and CO at 673 K. However, structural parameters derived from EXAFS analysis indicated that cerium remained in the CeO2 crystal structure regardless of gaseous environment. The average oxidation state of Pd was also affected by gaseous environment with an average oxidation state between 0 and 2+ for a stoichiometric mixture of NO and CO. Exposure of Pd particles to NO resulted in the formation of chemisorbed oxygen and/or a surface oxide layer that can be completely removed by exposure to CO at 573 K.