MgO-, SiO2-, and gamma-Al2O3-supported platinum clusters and particles (with average diameters ranging from 11 to 45 Angstrom) and zeolite-supported Ir-4 clusters (approximately 6 Angstrom in diameter) were characterized by extended X-ray absorption fine structure spectroscopy in the presence of H-2, O-2, ethene, propene, and ethane, as well as under conditions of alkene hydrogenation catalysis. The results indicate that under various atmospheres, the presence of adsorbates affects the smaller platinum clusters (11 Angstrom) on gamma-Al2O3 more substantially than the larger platinum particles (i.e., those greater than approximately 21 Angstrom in average diameter) on MgO or SiO2. When Pt/gamma-Al2O3 was exposed to H-2 the platinum morphology did not change, although the Pt-Pt bond distance increased. In contrast, when the same sample was exposed to O-2, complete oxidative fragmentation took place. This processes was reversed following subsequent treatment with H-2. Exposure to alkenes changed both the morphology and electron density (as indicated by X-ray absorption near-edge spectra) of the gamma-Al2O3- supported platinum clusters. Under conditions of alkene hydrogenation catalysis at room temperature, the electronic properties and the structure of the platinum clusters were found to depend on the reactant composition and the nature of molecules involved in the reaction process. The effects of the reactant gases on the smaller iridium clusters (Ir-4) were substantially less pronounced, apparently as a consequence of the extremely small number of atoms in each iridium cluster.