EPR spectroscopy has been used to study the adsorption and decomposition of molybdenum carbonyls (Mo(CO)/sub 6/) in the hydrogen and sodium forms of zeolite Y. In hydrogen (H)-Y, decomposition of adsorbed Mo(CO)/sub 6/ at 100/sup 0/C produces a paramagnetic species containing two equivalent Mo nuclei which is tentatively identified as a dinuclear carbonyl radical anion. Decomposition at higher temperatures produces initially a signal attributed to Mo/sup +/, and subsequently three distinct Mo/sup 5 +/ signals. Spin concentration measurements show that up to 50% of the Mo present in the zeolite is observed as Mo/sup 5 +/ following prolonged high-temperature activation. The interaction of oxygen, ammonia, and pyridine with the paramagnetic species has been investigated. In sodium (Na)-Y, no EPR signals are detected upon decomposition of adsorbed Mo(CO)/sub 6/ in vacuo, indicating that zerovalent molybdenum is not oxidized in the absence of zeolite protons. Exposure to oxygen of MoNa-Y activated at temperatures below 300/sup 0/C did cause appearance of Mo/sup 5 +/ signals. 9 figures, 1 table.