Isothermal kinetic studies on the reduction of NO by CO on Rh(1 1 1) single-crystal surfaces, performed under vacuum by using collimated effusive molecular beams, have provided information on the coverages and nature of the surface intermediates involved in that reaction under catalytic conditions. Three major conclusions were reached. First, the optimum rate of reaction is achieved when the steady-state coverages of NO and CO on the surface reach the 1:1 stoichiometric ratio. The surface coverages are controlled by a synergistic balance between the composition of the gas and the surface temperature: higher temperatures tend to require higher CO:NO ratios. Second, under optimum conditions the surface of the catalyst is mostly covered with atomic nitrogen. This nitrogen appears to cluster in islands, and further conversion to molecular nitrogen takes place preferentially at their periphery. Finally, the formation of molecular nitrogen under catalytic conditions is likely to involve the formation of a N-NO intermediate. Evidence for these conclusions is provided.