A low-cost and scalable metal–organic framework, aluminum formate (Al(HCOO)₃, ALF), exhibits high selectivity for CO₂ capture and non-cryogenic oxygen separation from air. Preferential adsorption of CO₂ over N₂ arises from size-selective separation driven by the subtle difference in their kinetic diameters. ALF also shows substantial O₂ uptake and excellent time-dependent O₂/N₂ selectivity in a range of 50–125 near dry-ice/solvent temperatures (≈190 K). Single-crystal X-ray diffraction and neutron powder diffraction, combined with density functional theory calculations, are employed to elucidate the underlying adsorption mechanisms.