The characterization of silica- and alumina-supported iridium catalysts prepared from Ir4(CO)12 is described. The techniques employed are infrared spectroscopy, temperature-programmed desorption, and gravimetric chemisorption measurements. Supported Ir4(CO)12 begins to decompose on heating in vacuo above 75 °C, and decomposition is complete at 350 °C. The major product of decomposition on silica is metallic iridium, whereas on alumina a fraction of the iridium becomes oxidized. Hydrogen reduction of both silica- and alumina-supported catalysts following decomposition of the parent complex in vacuo produces highly dispersed metallic iridium. From observed shifts in the infrared frequencies of adsorbed CO with coverage it is concluded that the iridium may be in the form of two-dimensional rafts, containing possibly 20 atoms or more.