We describe a cationic erbium-based material [Er-12(OH)(29)(H2O)(5)][O3SCH2CH2SO3](3.5)center dot 5H(2)O. As synthesized, the material is water stable and capable of complete organic anion exchange for a variety of a,?-alkanedicarboxylates. We chose these anions as initial examples of exchange and as an analog for pharmaceutical waste, some of which have a carboxylate functionality at neutral pH range. Free-floating and partially anchored organosulfonate anions reside between the cationic corrugated layers and allow for exchange. The structure also displays a reversible hydration event above 100 degrees C. Both the as-synthesized and the exchanged materials are characterized by a variety of analytical techniques.