Osmium carbonyls synthesized at low loadings in the supercages of zeolite NaX, including mononuclear osmium carbonyls and the following clusters, [HOs3(CO)(11)](-) [H3Os4(CO)(12)](-) and [Os5C(CO)(14)](2-) were identified by extended X-ray absorption fine structure and infrared spectroscopies. The samples were also characterized by Xe-129 NMR spectroscopy over the temperature range 100-310 K. The Xe-129 chemical shifts were greater for samples containing osmium carbonyls than for the bare zeolite over the entire temperature range. At the lowest temperatures, the chemical shifts representing xenon sorbed in the zeolites containing the osmium carbonyl clusters were essentially the same, but at temperatures close to room temperature, the chemical shift increased with decreasing size of the osmium carbonyl. The relatively large chemical shift observed for the smallest osmium carbonyl is consistent with the presence of Xe atoms in the cages with these mononuclear metal complexes, whereas Xe atoms barely fit into the cages with the larger [HOs3(CO)(11)](-), [H3Os4(CO)(12)](-), or [Os5C(CO)(14)](2-).