The metal organic host material [{Co-2(III)(bpbp)-(O-2)}(2)bdc](PF6)(4) (1 center dot 2O(2); bpbp(-) = 2,6-bis(N,N-bis(2-pyridylmethy)aminomethyl)-4-tert-butylphenolato; bdc(2-) = 1,4-benzene-dicarboxylato) displays reversible chemisorptive desorption and resorption of dioxygen through conversion to the deoxygenated Co(II) form [{Co-2(II)(bpbp)}(2)bdc](PF6)(4) (1). Single crystal X-ray diffraction analysis indicates that the host lattice 1 center dot 2O(2), achieved through desorption of included water guests from the as-synthesized phase 1 center dot 2O(2)center dot 3H(2)O, consists of an ionic lattice containing discrete tetranuclear complexes, between which lie void regions that allow the migration of dioxygen and other guests. Powder X-ray diffraction analyses indicate that the host material retains crystallinity through the dioxygen desorption/chemisorption processes. Dioxygen chemisorption measurements on 1 show near-stoichiometric uptake of dioxygen at 5 mbar and 25 degrees C, and this capacity is largely retained at temperatures above 100 degrees C. Gas adsorption isotherms of major atmospheric gases on both 1 and 1 center dot 2O(2) indicate the potential suitability of this material for air separation, with a O-2/N-2 selectivity factor of 38 at 1 atm. Comparison of oxygen binding in solution and in the solid state indicates a dramatic increase in binding affinity to the complex when it is incorporated in a porous solid.