The open-framework coordination polymer [Er(C9H9-O-6)(H2O)(2)]. 2.5H(2)O (C9H9O6 = cis,cis-1,3,5-cyclo-hexanetricarboxylate) has been synthesized under hydrothermal conditions. Each Er ion is coordinated to nine oxygen atoms belonging to two water molecules and five carboxylate ligands. Its 3-D polymeric structure possesses channels containing the guest water molecules that can be removed to afford a porous material [Er(C9H9O6)(H2O)(2)]. The metal-organic framework in [Er(C9H9O6)(H2O)(2)] can withstand the removal of I equiv of coordinated water.