The reactivity of adsorbed NO and CO has been investigated by combined in situ infrared (IR) and mass spectroscopy (MS) on Rh/SiO2, Ce-Rh/SiO2, and oxidized Rh/SiO2 at 298-423 K. Exposure of preadsorbed CO to a gaseous NO pulse on Rh/SiO2, and Ce-Rh/SiO2 resulted in the desorption of linear CO and formation of a low wavenumber NO- (1650-1700 cm-1) species on the reduced Rh site; exposure of preadsorbed CO on oxidized Rh/SiO2 to a gaseous NO pulse resulted in the displacement of the gem-dicarbonyl without formation of CO2 at 298-423 K. Increasing NO exposure time led to the desorption of all adsorbed CO and formation of NO+ species, a high wavenumber NO- (1740-1770 cm-1) species, and a bidentate nitrato species. Prolonged exposure of preadsorbed NO to CO resulted in displacement of the adsorbed NO’ and chemisorption of CO as gem-dicarbonyl and a linear CO on Rh+. Exposure of the catalyst to a mixture of NO-CO-He (1:1:3) at 373 K and 0.1 MPa resulted in the formation Of CO2. The formation of CO2 from the reaction of adsorbed CO with oxygen produced from dissociated NO was more rapid than the desorption of adsorbed CO. The linear CO adsorbed on a Rh site with oxidation state greater than + 1 may be a precursor towards CO2 formation. The increase in the rate Of CO2 formation may be related to the increased coverage and reactivity of adsorbed species in the presence of gaseous NO and CO.