Infrared spectroscopy was used to characterize the preparation of [HIr4(CO)(11)](-) from [Ir-4(CO)(12)] on the surface of MgO powder at room temperature and the conversion of the supported [HIr4(CO)(11)](-) into [Ir-6(CO)(15)](2-) by treatment in CO at 1 atm and 200 degrees C. During the preparation of [HIr4(CO)11](-), OH groups on the MgO were partially consumed, and surface carbonates formed; the surface reaction is analogous to that by which [Ir-4-(CO)(12)] is converted into [HIr4(CO)11](-) in basic solutions. The decarbonylated cluster has been characterized by extended X-ray absorption fine structure spectroscopy (as reported elsewhere) and modeled as Ir-4. When the decarbonylated clusters were recarbonylated, the infrared bands characteristic of MgO-supported [HIr4(CO)(11)](-) did not reappear; rather, new supported iridium carbonyls formed. Chemisorption of H-2, CO, and O-2 On the decarbonylated clusters was measured at room temperature. More O-2 was chemisorbed on the MgO-supported Ir-4 than on metallic iridium particles, whereas the uptake of H-2 and of CO on the iridium clusters was less than that observed for metallic iridium particles. All the results imply that the decarbonylated clusters should be considered to be quasi-molecular and not metallic.