Clusters of rhodium and of iridium were synthesized in the cages of the sodium form of the faujasite zeolite Y by decarbonylation of zeolite-supported [Rh-6(CO)(16)] or [Ir-4(CO)(12)] in H-2 at 200 or 300 degrees C, respectively. Larger clusters (aggregates) of rhodium or iridium consisting of 10 to 20 atoms each on average were formed by treatment of zeolite-supported [Rh-6(CO)(16)] or [Ir-6(CO)(16)] in H-2 at 250 or 300 degrees C. The samples were characterized by extended X-ray absorption fine structure (EXAFS) spectroscopy under vacuum and in (a) H-2, (b) propene, and (c) H-2/propene mixtures (with catalytic hydrogenation taking place). The EXAFS data indicate that the clusters or aggregates were intact, with the metal frames changed undetectably in the presence of any of the gases, even during catalysis and at temperatures up to 140 degrees C. In the presence of propene only, rhodium clusters showed changes in their X-ray absorption near-edge spectra, but the iridium clusters did not. When propene was present with H-2 during catalysis, the near-edge spectra were essentially the same as those characterizing the metal clusters (or aggregates) in H-2 only. The data are consistent with the inference that the metal clusters, approximated as predominantly Ir-4 or Rh-6 (the latter not fully decarbonylated) are catalytically active themselves, as are larger aggregates of those metals.