Iridiun clusters were prepared on gamma-Al2O3 that had been calcined at 400 degrees C. When supported [Ir-4(CO)(12)], the precursor, was treated in H-2 at various temperatures, it was decarbonylated, with aggregation taking place to various degrees to give a family of supported clusters and aggregates with average diameters in the range of about 9 to 33 Angstrom, as determined by extended X-ray absorption fine structure (EXAFS) spectroscopy. Similarly, samples were decarbonylated in He, and the degree of aggregation of the iridium was less than in H-2 under comparable conditions. Infrared spectra show that decarbonylation of the supported [Ir-4(CO)(12)] in the presence of H-2 was not reversible and was accompanied by aggregation of the iridium; infrared spectra of CO adsorbed on the decarbonylated samples were characterized by a band at 2058 cm(-1), typical of terminal CO adsorbed on iridium particles. The gamma-Al2O3-supported iridium clusters and aggregates were used to catalyze toluene hydrogenation at 60 degrees C and 1 atm. Samples with nearly the same Ir-Ir first-shell coordination number (nearly the same dispersion) were characterized by essentially the same catalytic activity, independent of whether the decarbonylation was carried out in He or H-2. The catalytic activity for the structure-insensitive toluene hydrogenation reaction was found to increase with increasing average cluster or aggregate diameter and to be greater for iridium on the calcined gamma-Al2O3 than for iridium on uncalcined gamma-Al2O3.