A reaction chamber attached to a UHV surface analysis system is used to study kinetics of HCN synthesis on polycrystalline Rh for temperatures between 500 and 1600 K with CH₄ and NH₃ pressures between 0.05 and 10 Torr. AES is used to show that the surface is clean before reaction, and AES and TPD are used to examine surface residues after reaction. The rate of HCN production attains a maximum near a 1:1 reactant mixture, and exceeds 10¹⁸ molecules/cm² sec at high temperatures. The selectivity (fraction of NH₃ reacting to form HCN) is greater than 90% at high temperatures at a total pressure of 1 Torr. The rate of HCN production is proportional to P_CH4, and N₂ formation is strongly inhibited by CH₄. Examination of rhodium foils by SEM after reaction shows that the surface is faceted into predominantly (100) planes, and grain boundaries are pitted. Reaction is shown to occur by NH₃ or its fragments reacting with surface carbon layers because approximately one monolayer of carbon is found on the reactive surface while multilayers of carbon create a totally unreactive surface. This carbon also explains why the NH₃ decomposition reaction is suppressed compared to that on clean Rh. A model in which surface carbon is a reactant which also blocks reaction sites is shown to give a quantitative fit to rates of HCN and N₂ production. The high reaction probability of CH₄ (˜10⁻²) is surprising because of its low surface reactivity on Rh. It is argued that the surface reaction between NH₃ and CH₄ fragments is the major reaction under industrial HCN synthesis conditions and that no homogeneous processes are necessary to explain high yields of HCN.