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The Zr‐based Metal Organic Framework (MOF) UiO‐66(Zr) is widely employed owing to its good thermal and chemical stabilities. Although the long‐range structure of this MOF is preserved in the presence of water during several days, little is known about the formation of defects, which cannot be detected using diffraction techniques. We apply here 17O solid‐state NMR spectroscopy at 18.8 T to investigate the reactivity of UiO‐66, through the exchange of oxygen atoms between the different sites of the MOF and water. For that purpose, we have selectively enriched in 17O isotope the carboxylate groups of UiO‐66(Zr) by using it with 17O‐labeled terephthalic acid prepared using mechanochemistry. In the presence of water at 50 °C and a following dehydration at 150 °C, we observe an overall exchange of O atoms between COO− and μ3‐O2− sites. Furthermore, we demonstrate that the three distinct oxygen sites, μ3‐OH, μ3‐O2− and COO−, of UiO‐66(Zr) MOF can be enriched in 17O isotope by post‐synthetic hydrothermal treatment in the presence of 17O‐enriched water. These results demonstrate the lability of Zr−O bonds and the reactivity of UiO‐66(Zr) with water. [ABSTRACT FROM AUTHOR]