We report a comparison of MgO-supported Rh-6 and MgO-supported Ir6 as catalysts for ethene hydrogenation. The catalysts were prepared by decarbonylation of hexanuclear metal carbonyl precursors on the support. Extended X-ray absorption fine structure and infrared spectroscopies were used to characterize the structures of the working catalysts and the adsorbates formed on them during catalysis. The data identify the octahedral clusters as the catalytically active species. Supported Rh-6 is 1-2 orders of magnitude more active than supported Ir-6 for ethene hydrogenation; this difference in activity is consistent with that observed for conventional rhodium and iridium catalysts consisting of bulklike particles of metal on SiO2. The results demonstrate that the influence of metal composition on catalytic activity extends from quasi-molecular metal clusters to metallic particles.