A study of the characteristics and catalytic behaviour of silica-supported Rh–Ir catalysts has been carried out. Catalysts spanning the spectrum of possible metal compositions were used. Both chemisorption and X.p.s. results suggest a modest Rh enrichment of the surface relative to bulk composition. Hydrogenation activity was characterized by cyclopropane hydrogenation and pure Rh shows activity orders-of-magnitude greater than that of pure Ir. The hydrogenation turnover frequency (normalized by assuming a hydrogen-to-metal surface atom ratio of one) shows a slight maximum on the Rh-rich catalysts which may reflect an electronic effect or be the result of selective chemisorption. The hydrogenolysis reactions are most rapid on Rh and the turnover frequencies decrease in a non-linear manner from Rh to Ir. Even though the absolute rates are different, both hydrogenation and hydrogenolysis reactions have reactivity patterns qualitatively similar to the Ni–Cu system. The results for both systems can be interpreted on the basis of different numbers of atoms in the ensembles which constitute the site, larger ensembles being required for the hydrogenolysis reaction. The relative rate of H–D exchange of methane and the pattern of the rates as a function of catalyst composition indicate that methane desorption is not the rate-limiting step in ethane hydrogenolysis.