Reaction of Cr(CO)(6) with trimesic acid in DMF affords the metal-organic framework Cr-3(BTC)(2)center dot nDMF (BTC3- = 1,3,5-benzenetricarboxylate), which is isostructural to Cu-3(BTC)(2)center dot 3H(2)O. Exchanging DMF for methanol and heating at 160 degrees C under dynamic vacuum for 48 h results in the desolvated framework Cr-3(BTC)(2). Nitrogen gas adsorption measurements performed at 77 K revealed a type I isotherm, indicating BET and Langmuir surface areas of 1810 and 2040 m(2)/g, respectively. At 298 K, the O-2 adsorption isotherm for Cr-3(BTC)(2) rises steeply to a capacity of 11 wt % at 2 mbar, while the corresponding N-2 adsorption isotherm displays very little uptake, gradually rising to a capacity of 0.58 wt % at 1 bar. Accordingly, the material displays an unprecedented O-2/N-2 selectivity factor of 22. Deoxygenation of the sample could be accomplished by heating at 50 degrees C under vacuum for 48 h, leading to a gradually diminishing uptake capacity over the course of 15 consecutive adsorption/desorption cycles. Infrared and X-ray absorption spectra suggest formation of an O-2 adduct with partial charge transfer from the Cr-II centers exposed on the surface of the framework. Neutron powder diffraction data confirm this mechanism of O-2 binding and indicate a lengthening of the Cr-Cr distance within the paddle-wheel units of the framework from 2.06(2) to 2.8(1) angstrom.