Supported clusters represented as Ir-4 on gamma-Al2O3 were prepared by treatment of gamma-Al2O3-supported [Ir-4(CO)(12)] in He at 300 degrees C. The clusters were decarbonylated with near retention of the tetrahedral cluster frame, as indicated by extended X-ray absorption fine structure (EXAFS) spectra determining a first-shell Ir-Ir coordination number of 3.04 with a distance of 2.682 Angstrom. X-ray absorption near-edge structure data characterizing the catalysts and Ir metal indicate the electron deficiency of the supported clusters, and the EXAFS data indicate an Ir-O distance of about 2.1 Angstrom, which is consistent with bonding of cationic Ir in the clusters to oxygen of the support. The supported Ir-4 clusters were found to be catalytically active for the structure-insensitive toluene hydrogenation at temperatures in the range 60 to 100 degrees C, with the activity being almost the same as that of Ir-6/gamma-Al2O3 and an order of magnitude less than that of Ir aggregates of about 50 atoms each, on average, supported on gamma-Al2O3. The Ir4/gamma-Al2O3 catalyst was stable in operation in a flow reactor, and, consistent with this observation, EXAFS results characterizing the used catalyst indicated that the cluster nuclearity was essentially unchanged after catalysis. The low activity of the supported Ir-4 clusters relative to that of metallic Ir particles indicates that the concept of structure insensitivity is no longer valid for clusters as small as Ir-4 On the metal oxide support.