Ideal adsorption solution theory (IAST) was used to predict the equilibrium adsorption behavior of Cu2.7-MFU-4l exposed to a binary O2/N2 mixture and extract O2/N2 adsorption selectivities as a function of O2 concentration (see the Supporting Information for details). For a binary mixture containing 21% O2, the O2/N2 selectivity is 10 at 298 K, which would correspond to 72% adsorbed phase purity (74% at 288 K; Figure ). It should be noted that one of the assumptions of IAST is that there is a homogeneous distribution of guests within the pores of the material, an assumption that may not hold in the present case, where electron transfer is a driving force in O2 binding but not in the case of N2 binding. In a realistic scenario where O2 preferentially clusters around the copper(I) sites, IAST is expected to overestimate actual selectivity values. However, breakthrough analysis using dry and humid compressed air streams indicate O2 does indeed bind selectively over N2 under dynamic conditions (see below). Interestingly, predicted N2/Ar and O2/Ar selectivities suggest that Cu2.7-MFU-4l may also be appropriate for removing N2 and O2 impurities in Ar purification (Figure ).