Molecular clusters of rhodium were synthesized in the cages of NaY zeolite by decarbonylating supported rhodium carbonyls, which were predominantly [Rh-6(CO)(16)] prepared by carbonylation of adsorbed [Rh(CO)(2)(acac)] at 125 degrees C The samples were characterized by infrared and X-ray absorption fine structure spectroscopies. [Rh-6(CO)(16)] formed at 125 degrees C, with a higher yield observed for the clusters in the uncalcined zeolite than for those in the calcined zeolite. Rhodium clusters formed by decarbonylation of [Rh-6(CO)(16)] in calcined NaY zeolite in the presence of He at temperatures of 200, 250, or 300 degrees C were characterized by Rh-Rh coordination numbers of approximately 3.7, indicating that the octahedral metal frame of the [Rh-6(CO)(16)] precursor remained nearly intact. When the decarbonylation of [Rh-6(CO)(16)] took place in the presence of H-2, partially decarbonylated rhodium clusters formed at 200 degrees C, having a Rh-Rh coordination number of about 3.6. Higher-temperature treatments in H-2 resulted in the sintering of rhodium. When [Rh-6(CO)(16)] in uncalcined NaY zeolite was decarbonylated in the presence of He, it led to the formation of particles with a Rh-Rh coordination number of 7.4, indicating that the rhodium aggregated and migrated through the zeolite pores. Thus, the water in the zeolite that favors the formation of [Rh-6(CO)(16)] in high yields causes a loss of the cluster framework during subsequent decarbonylation. The clusters that had been partially decarbonylated in H-2 at 200 degrees C were recarbonylated to reconstitute [Rh-6(CO)(16)], but the clusters that had been formed by decarbonylation in He at the same temperature could not be reversibly recarbonylated. The important result is that fully or partially decarbonylated rhodium clusters, consisting of about six atoms can be formed by the decarbonylation of [Rh-6(CO)(16)] in the presence of He or H-2, respectively, and the decarbonylation chemistry is dependent on the zeolite water content.