Reversible benzene dicarboxylate/2-bromobenzene dicarboxylate ligand exchange has been studied in the cubic metal organic framework MOF-5. Significant exchange (up to similar to 50%), with continuous compositional variation, was observed using ex-situ H-1 NMR following treatment over similar to 6 h at similar to 85 degrees C in 10-40 mM ligand solutions. Exchange occurred without significant structural degradation as characterized by X-ray diffraction, nitrogen adsorption, and scanning electron microscopy. Solid-state C-13 NMR was used to show that exchanged ligands were incorporated into the framework lattice and not simply adsorbed within the pores. Exchange was found to be sensitive to the small free energy changes caused by the ligand concentration in the exchanging solution indicating that exchange is energetically nearly degenerate. This demonstration of reversible, nearly isoenergetic exchange indicates that mixed ligand MOFs could be developed as dynamic combinatorial chemical systems.