In its crystalline state, a dinuclear Cu-based metallocycle discriminates between the three isomers of xylene with liquid-phase selectivity in the order p-xylene >> m-xylene >> o-xylene. This selectivity holds over a wide concentration range, with p-xylene concentrations as low as 5%. Single-crystal X-ray diffraction and gas chromatography further indicate that the metallocyclic host extracts trace amounts of p-xylene from commercially pure o-xylene (>= 99%); using NMR spectroscopy, we show that the metallocycle exhibits exclusive selectivity for p-xylene. Crystallographic studies show that the selectivity is based on the size and shape of the guest in combination with the flexibility of the host.