The oxidation of CO by O/sub 2/ or NO over Rh(111) and Rh(100) single crystals has been studied in a high-pressure reaction-high-vacuum surface analysis apparatus. Steady-state catalytic activity as a function of temperature and partial pressures of CO, O/sub 2/, and NO has been measured. The CO-O/sub 2/ reaction was found to be insensitive to the structure of the surface as evidenced by the identical rates, activation energies, and partial pressure dependencies measured on the two-single-crystal surfaces. Both surfaces deactivated at high O/sub 2/ partial pressures to the formation of a near-surface oxide (probably Rh/sub 2/O/sub 3/) which is catalytically inactive for this reaction. The deactivation occurred at a slightly lower O/sub 2/ partial pressure on Rh(100), likely due to the relative ease of oxygen diffusion through this surface leading to more rapid bulk oxidation. Unlike the CO-O/sub 2/ reaction, CO oxidation by NO was very sensitive to the geometric structure of the surface. This behavior was evident in all kinetic parameters measured on the two surfaces. The kinetic data obtained on Rh(111) agree quantitatively with a surface chemistry model previously developed for this reaction. The model predicts that the rate is limited by the formation of N/sub 2/ frommore » the recombination of adsorbed N atoms. and that the surface, under steady-state reaction conditions, is covered largely with adsorbed N atoms and smaller amounts of adsorbed NO. On Rh(100) the data are consistent with a reaction rate limited by the formation of N/sub 2/ from the surface reaction of adsorbed NO and N atoms.« less