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Twofamilies of metal–organic frameworks (MOFs), MIL-88and MIL-101 built by trinuclear transition metal (TM) clusters (TM= Cr, Fe, or Sc), have been known for several years, but their synthesesare often reported separately. In fact, these MOFs are polymorphs,or framework isomers: they are assembled from the same metal secondarybuilding units and organic linkers, but the connectivity of thesecomponents differs. Here we report for the first time the synthesisof the vanadium MOF MIL-88B(V) and compare its synthesis parametersto those of MIL-47(V) and the recently reported MIL-101(V). The propertiesof MIL-88B(V) and MIL-101(V) are remarkably different. MIL-88B(V)can “breathe” and is responsive to different solvents,while MIL-101(V) is rigid and contains mesoporous cages. MIL-101(V)exhibits the highest specific surface area among vanadium MOFs discoveredso far. In addition, both MIL-88B(V) and MIL-101(V) transform to MIL-47at higher temperatures. We have also identified the key synthesisparameters that control the formation of MIL-88B(V), MIL-101(V), andMIL-47: temperature, time, and pH. This relates to the rate of reactionbetween the metal and linkers, which has been monitored by ex situX-ray powder diffraction and V K-edge X-ray absorptionspectroscopy during MOF synthesis. It is therefore important to fullystudy the synthesis conditions to improve our understanding of frameworkisomerism in MOFs. [ABSTRACT FROM AUTHOR]