Supported [HIr4(CO)(11)](-), prepared by adsorption of [Ir-4(CO)(12)] from n-pentane solution onto MgO powder that had been partially dehydroxylated in vacuum at 400 degreesC, was identified by infrared and extended X-ray absorption fine structure (EXAFS) spectroscopies. The clusters were decarbonylated to various degrees by treatment in He or H-2 and characterized by infrared and EXAFS spectroscopies. When the decarbonylation was carried out in H-2, the iridium aggregated; infrared data show that the CO ligands reacted with H-2, resulting in the formation of water and hydrocarbons. When the decarbonylation was carried out in He, the tetrahedral cluster frame remained unchanged; the fury decarbonylated clusters, modeled as Ir-4 tetrahedra, had an Lr-Ir first-shell coordination number of 3.0 +/- 0.1 at a distance of 2.67 +/- 0.01 Angstrom. The partially decarbonylated tetrairidium clusters chemisorbed hydrogen. Temperature-programmed desorption (TPD) data show that the strength of bonding of hydrogen to the clusters increased with increasing cluster decarbonylation. The completely decarbonylated clusters were characterized by low chemisorption capacities (H/Ir and CO/Ir values, e.g. 0.27 and 0.39, respectively). Chemisorption and EXAFS data show that the amount of hydrogen or CO chemisorbed on Ir-4 increases with increasing size of the iridium clusters and aggregates, consistent with a support effect that is maximized for the smallest clusters, Ir-4. Data representing the catalytic hydrogenation of toluene show that the rate depends on the amount of hydrogen on the clusters - and not just on the degree of decarbonylation - consistent with the suggestion that iridium atoms free of CO Ligands are not sufficient for catalysis to proceed. Toluene adsorbed on the support is implicated in the catalysis.