We have examined the reaction of NO and CO over a Rh(III) catalyst under conditions of both low (0.8 Torr) and high (9 Torr) NO pressures. Our results show that the same three products are formed (CO{sub 2}, N{sub 2}O, and N{sub 2}) regardless of the NO pressure employed. However the selectivity of the reaction and the E{sub a} for product formation are profoundly affected when the NO pressure is lowered by tenfold. For high NO pressures the selectivity is insensitive to either NO pressure (1 Torr < P{sub NO} < 40 Torr) or reaction temperature (T < 673 K). In sharp contrast to this high-pressure behavior, when the NO pressure is lowered to 0.8 Torr, then the selectivity is a strong function of reaction temperature, with N{sub 2}O being the major product below 635 K and N{sub 2} the major product above 635 K. It is exactly this switch over in selectivity at elevated temperature that has always been seen over low-loaded Rh/Al{sub 2}O{sub 3} catalysts but until this time not reported for single-crystal catalysts. On the basis of our new results we can say that Rh(III) has the characteristic selectivity behavior of practical Rh/Al{sub 2}O{sub 3} catalysts. Thesemore » results are encouraging because they show that the mechanistic insight we gain over Rh(III) is more broadly applicable to supported catalysts than was previously thought. 20 refs., 4 figs., 1 tab.« less