Rhodium is well known for its unique ability to reduce NO to N@sub 2@. It is this property that makes it indispensable in the three way catalysts used in cars to clean their exhausts. Former studies in this laboratory using molecular beams have shown that the catalytic reduction of NO by CO takes place at the periphery of surface islands of adsorbed nitrogen atoms, and most likely involves the formation a N-NO intermediate. Here, results from a kinetic study on the conversion of N@sub 2@O + CO mixtures on Rh(111) surfaces are presented. It was found that the overall behaviour is similar to that of the NO + CO reaction. The reaction rate with both nitrogen oxides reaches a maximum near 500 K for stoichiometric beams, but in the case of N@sub 2@O the rate-limiting step is the formation of CO@sub 2@, not the production of N@sub 2@ as when NO is used. These results will be contrasted with our previous work on the NO + CO and CO + O@sub 2@ systems to get an overall picture of the elementary steps involved in the cleaning of car exhausts.