Unlike bulk ZrO 2 , Zr-MOFs provide two pathways for creating defects: breaking metal-ligand (Zr-O-L) or metal-oxygen-metal (Zr-O-Zr) bonds. Missing linker defects do not lower Zr valency; Zr-O-L bonds are replaced by Zr-O-X bonds, where X is a charge-compensating ion. The only crystallographically resolved missing-linker defect structure, in UiO-66 ({Zr 6 } interlinked by terephthalate ligands), is characterized by terminal oxygen groups, which were identified as H 2 O groups with an OH -group H-bonded to the µ 3 -OH group of the {Zr 6 } cluster for charge compensation. Despite numerous catalytic studies reported for Zr-MOFs, the role of oxygen vacancies still needs to be explored. At temperatures between 250 and 300 • C, dehydration of {Zr 6 } clusters occurs, which is believed to create undercoordinated ZrO 7 polyhedra. The percentage of low-valent ZrO 7 sites varies with the degree of dehydration; removing one or two µ 3 -OH groups yields {Zr 6 } cores with compositions of Zr 6 O 5 (OH) 2 and Zr 6 O 6 containing 3 and 6 Zr cus sites, respectively. We surmise that these Zr cus sites act as Lewis acidic open metal sites, improving the catalytic activity of the {Zr 6 } clusters.