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The nature and energy of the reactions between calcium vapor and the internal surfaces of the metal-organic framework (MOF) NU-1000 have been studied by adsorption microcalorimetry, low energy He+ ion scattering spectroscopy (LEIS), X-ray photoelectron spectroscopy (XPS), and Kohn-Sham density functional theory (DFT). NU-1000 is one of the most stable MOFs with transition-metal-oxide nodes, and thus it is of interest as a potential catalyst or catalytic support when modified with other metals. The reaction heats of Ca with NU-1000 are high below 2 monolayers (ML) Ca coverage (570-366 kJ/mol), attributed (based on DFT) to Ca reacting first with free benzoic acid functionalities or water impurities, then with H2O and OH groups on the Zr6 nodes to produce Ca(OH)2 clusters. With higher Ca doses, the heat of Ca reaction decreases asymptotically to the sublimation enthalpy of bulk Ca (178 kJ/mol), attributed to the formation of Ca(solid) nanoparticles on the external surface, which only occurs after all of the H2O and OH groups are titrated deeply enough (∼20 nm) such that slow Ca diffusion prevents further reaction.