Rh surface states and their adsorbates during the NO-CO reaction have been characterized by the in situ infrared (IR) coupled with temperature-programmed reaction (TPR) technique. The TPR profiles of adsorbates and CO2 show that Rh surface states and their adsorbates are governed by the redox reaction cycle of NO-CO. Adsorbed oxygen from dissociated NO oxidizes Rh-0 to Rh+; adsorbed CO reduces Rh+ to Rh-0. The extent of oxidation and reduction of Rh-0/Rh+ is in part reflected in the intensity of the adsorbates residing on these sites (i.e., Rh+(CO)(2), Rh-0-CO, Rh-NO+, and Rh-0-NO-). An increasing NO/CO ratio shifts the TPR profiles of Rh+(CO)(2), NO conversion, and light-off to higher temperatures. The results reveal that a high NO/CO ratio or high concentration of oxidant enhances the extent of oxidation of Rh-0 to Rh+, resulting in low catalyst activity for NO reduction. Keeping the Rh surface in the Rh-0 state by a low NO/CO ratio decreases the Rh+(CO)(2) intensity and shifts the light-off to a lower temperature. O-2, H-2, and C3H8 present in simulated gas compete with NO and CO for the Rh site, lowering NO reduction activity.