Temperature programming of NO and C2H4 coadsorbed on Rh(111) gives rise to the desorption of a number of gases. Where H-2, H2O, CO2 and N-2 are the main products at low C2H4 coverages, significant amounts of HCN, CO and NO evolve at higher C2H4 coverages. Static SIMS indicates the formation of a large supply of adsorbed CN species, part of which desorbs as HCN, while the remainder decomposes and is responsible for delayed formation of N-2. For the highest C2H4 coverages the majority of the initially adsorbed NO desorbs as HCN.