NaY zeolite-supported rhodium carbonyls, with the predominant species being [Rh-6(CO)(16)], were prepared by carbonylation of adsorbed [Rh(CO)(2)(acac)] at 125 degrees C. The zeolite-supported rhodium carbonyl clusters were treated in He or in H-2 at 200, 250, or 300 degrees C to remove the carbonyl ligands. When the decarbonylation took place in He, the resultant clusters had Rh-Rh coordination numbers (determined by extended X-ray absorption fine structure spectroscopy) between 3.5 and 3.9, with hardly any evidence of Rh-C contributions, indicating that the rhodium cluster frames were nearly intact and that decarbonylation was nearly complete. In contrast, when the treatment took place in Hz, partially decarbonylated rhodium clusters and aggregates formed, having Rh-Rh coordination numbers between 3.8 and 6.7; the rhodium aggregation increased with increasing temperature. The clusters formed by treatment in He at 200, 250, or 300 degrees C all had similar catalytic activities for toluene hydrogenation. In contrast, the catalytic activities of the clusters and aggregates (partially decarbonylated at temperatures <300 degrees C) formed in the presence of Hz showed a strong dependence on treatment temperature, which is explained by inhibition by the remaining CO ligands and by cluster and aggregate size effects, with the aggregates being more active than the smaller clusters. Comparison of the data with those reported separately for rhodium clusters in NaX zeolite shows that NaY and NaX zeolites were nearly equivalent as supports. Comparison of the data with separately reported data indicates that the rhodium clusters are approximately the same in activity as iridium clusters of nearly the same size in NaY zeolite.