Temperature programmed reaction spectroscopy and secondary ion mass spectrometry (SIMS) have been applied to study reactions of NO and C2H4 coadsorbed on Rh(111). As expected, H2O, CO2, and N-2 are the main products at low C2H4 coverages, but at higher coverages H-2, HCN, CO, and NO, and even some C2N2 evolve as well. 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 nitrogen atoms in the initially adsorbed NO desorbs as HCN.