Stable zeolite-like microporosity, even in the absence of guest molecules, is exhibited by a lanthanide–benzenedicarboxylate (BDC) framework, which was synthesized by the copolymerization of TbIII and 1,4-benzenedicarboxylic acid. The open Tb–BDC framework adopts an expanded PtS network structure (see diagram; Tb: open circles; C: full circles; lines connecting Tb to C represent O atoms, and those connecting C atoms are benzene rings) and is capable of reversible gas and liquid sorption.