The prospect of using O2-selective adsorbents for energy-efficient air separations has garnered research attention for decades, beginning with early studies of O2 binding in molecular cobalt(II) complexes. A renaissance in this area has occurred within the previous decade with the discovery that certain porous, microcrystalline metal-organic frameworks (MOFs) featuring coordinatively unsaturated iron(II), cobalt(II), and chromium(II) sites can bind O2 via electron transfer mechanisms that give rise to excellent selectivities for O2 over typically redox-inactive N2. Importantly, air separations using cation-exchanged zeolites that selectively adsorb N2 over O2 (and Ar) are already used in industry to supplement cryogenic distillation for applications where O2 purities <95% are sufficient (e.g., for medicinal use). As such, in particular for medium to small-scale applications, infrastructure is in place that could in principle be adapted to implement separations technology using O2-selective adsorbents. A porous adsorbent capable of selectively capturing O2 over N2 and the other components of air, such as water, could be used to produce high purity O2 in a process requiring no air pre-treatment (other than removal of particulate matter) and thus fewer unit operations than cryogenic distillation or N2-selective adsorptive separations (Figure ). Furthermore, in principle, far less adsorbent would be required to process a given quantity of air than is needed for air separations using N2-selective zeolites, given that the concentration of O2 (21%) in air is much less than the concentration of N2 (78%). Consequently, the capital and energy expenditures required for air separations using an O2-selective adsorbent could be significantly less than required for cryogenic distillation and current adsorptive separations. However, the majority of O2-selective MOFs studied to date adsorb appreciable O2 only at sub-ambient temperatures or exhibit poor stability to repeated cycling. Additionally, these materials feature coordinatively unsaturated metal sites that can also serve as strong binding sites for water. Importantly, none of the corresponding studies have examined adsorbent O2 selectivity and capacity in the presence of water vapor.