Summary: A partially Co,-exchanged zeolite X was thermally treated to simulate the effect of decay heat on the leachability of extraframework Co,. To have a mechanistic insight into thermal effect, X-ray diffraction, scanning electron microscopy, ,Al magic angle spinning nuclear magnetic resonance spectroscopy, and Co K-edge X-ray absorption spectroscopy were employed with leaching tests. Although thermal treatment at ≤ 600 °C did not lead to the collapse of zeolite framework, it removed H,O molecules from the coordination shell of extraframework Co,, which in turn changed its coordination structure in a way to strengthen the interaction between Co, and the lattice oxygens. In leaching tests, the sample treated at higher temperature for a longer period showed less remobilized Co, by forming a Co(OH),-like surface precipitate and a Co hydrotalcite-like phase. Notably, the formation of the latter phase indicated the abstraction of the framework Al, the extent of which increased with the treatment temperature and duration. Two mechanisms, the concurrent extraction of Al with Co, remobilization and the hydrolysis-promoted Al abstraction, were proposed to account for thermally promoted dealumination. This study suggests that the exposure of Co,-exchanged zeolite X to decay heat lessen the risk of extraframework Co, to be reintroduced into groundwater.