Changes in the activity and selectivity of Rh and Rh-Ce catalysts supported on Al2O3 and SiO2 in the NO + CO reaction have been measured as a function of pretreatment. Rh-Ce/Al2O3 had very low activity following a pretreatment in H-2, which increased by a factor of approximately 10 after heating in NO, and increased further in CO and in NO + CO. Rh-Ce catalysts also had very low N2 selectivity vs N2O after treatments in H-2, CO, and NO + CO, but not after heating in air or NO, and had a lower activation energy for NO reduction than Rh catalysts. The activities of Rh/Al2O3 and Rh/SiO2 catalysts correlate well with changes in microstructures. An increased dispersion produced an increased activity at low temperatures except for the air pretreatment which oxidized the Rh and produced the lowest activity. Selectivity to N2 increased with increasing temperature below 300-degrees-C up to 100% for all pretreatments. All changes on alumina supported catalysts were reversible. The activity of Rh/SiO2 decreased after heating in NO + CO because of volatilization of Rh.