The chemistry of iridium clusters formed from [Ir-4(C0)(12)] on basic MgO surfaces was investigated with infrared and extended X-ray absorption fine structure (EXAFS) spectroscopies. The chemistry of iridium carbonyls on MgO is comparable to that of iridium carbonyls in basic solutions. The adsorption of [Ir-4(CO)(12)] on MgO with high, intermediate, and low surface hydroxyl group concentrations led to the formation of structures that are suggested on the basis of infrared spectra to be [Ir-4(CO)(11)COH{O}], [HIr4(CO)(11)]-, and [Ir-8(CO)(22)](2-), respectively, where {O} represents oxygen of the MgO surface; Decarbonylation of the iridium carbonyl clusters on MgO led to the formation of iridium clusters. The reaction of [Ir-4(C0)(12)] with highly dehydroxylated MgO followed by treatment in flowing He at 325 degrees C for 2 h and flowing H-2 at 300 degrees C for 2 h at 1 atm led to formation of clusters that are modeled on the basis of EXAFS spectra as predominantly Ir-4 tetrahedra. The reaction of [Ir-4(CO)(12)] with highly hydroxylated MgO followed by treatment in flowing He at 325 degrees C for 2 h and flowing H-2 at 300 degrees C for 2 h at 1 atm led to the formation of larger iridium clusters, with average diameters of about 12 Angstrom. Treatment of the Ir clusters on MgO in D-2 Saturated with D2O led to the formation of Ir particles > 100 Angstrom in diameter. The EXAFS data also provide evidence of the structure of the metal-metal oxide interface. The results suggest how variations of metal-support interactions resulting from changes in surface OH group concentration can be used to prepare supported metal clusters of controlled nuclearity.