The industrial separation of xylene isomers is a challenge due to their similarity in size, shape, density, boiling point, and polarity. Xylene isomers have been separated using zerodimensionally porous crystals composed of a Cu-based metallocycle, with selectivity in the order p-xylene ≫ m-xylene > o-xylene. The selectivity trend follows that of increasing kinetic diameters of the xylenes. Crystallographic studies show that the flexible host transforms from one phase to another to adapt to the inclusion of xylenes.