Rates for isotopically labelled molecular nitrogen production during the isothermal steady-state reduction of NO to N-2 by CO on Rh(1 1 1) surfaces were measured by using a molecular beam technique. The replacement of surface N-14 by N-15 upon switching the isotopic nitrogen label in the NO (from (NO)-N-14 to (NO)-N-15) was determined to occur via the exclusive formation of (NN)-N-14-N-15; no (NN)-N-14-N-14 was detected in these experiments. This result provides direct kinetic evidence for a mechanism for molecular nitrogen production involving the formation of an N-NO intermediate. The implications for the design of catalytic converters are briefly discussed.