Neutral and anionic iridium carbonyl clusters were synthesized from [Ir(CO)(2)(acac)] adsorbed on gamma-Al2O3 and on MgO surfaces, respectively. The samples were characterized as CO hydrogenation catalysts and found to have low activities at 21.4 atm, 250 degrees C, and H-2/CO molar ratios of 1-3. Both catalysts gave Schulz-Flory distributions of hydrocarbon products. At low conversions (<2%), the gamma-Al2O3-supported catalyst gave mainly paraffins, whereas the MgO-supported catalyst gave hydrocarbon products with relatively high olefin to paraffin ratios. The gamma-Al2O3-supported catalyst that had been used for 1 day in a flow reactor was dark gray, indicating the formation of metallic iridium aggregates. However, the used MgO-supported catalyst was reddish brown, and its infrared spectrum showed that the predominant iridium-containing species was [Ir-6(CO)(15)](2-), which could be extracted from the surface by cation metathesis.