The demand for Xe/Kr separation continues to grow due to the industrial significance of high-purity Xe gas.Current separation processes rely on energy intensive cryogenic distillation. Therefore,l ess energy intensive alternatives,s uch as physisorptive separation, using porous materials,a re required. Herein we showt hat an underexplored class of porous materials called hybrid ultra-microporous materials (HUMs) affords new benchmark selectivity for Xe separation from Xe/Kr mixtures.The isostructural materials,CROFOUR-1-Ni and CROFOUR-2-Ni, are coordination networks that have coordinatively saturated metal centers and two distinct types of micropores,o ne of which is lined by CrO 4 2À (CROFOUR) anions and the other is decorated by the functionalizedo rganic linker.T hese nets offer unprecedented selectivity towards Xe.M odelling indicates that the selectivity of these nets is tailored by synergy between the pore sizea nd the strong electrostatics afforded by the CrO 4 2À anions.Xenon (Xe) and Krypton (Kr) separation is of industrial interest because these gases present either commercial value as acommodity or an environmental threat. [1] High-purity Xe gas is valuable because of its applications in imaging, lighting, lasers,and medical science.The existing technology for Xe/Kr separation uses cryogenic distillation, which is energetically demanding and laborious.T his was reflected in our recent economic analysis,w hich suggests that metal-organic framework (MOF) based separation at room temperature could be more cost effective than cryogenic distillation in the context of nuclear reprocessing plants.The radioactive 85 Kr and 133 Xe can be introduced to the atmosphere during nuclear reprocessing operations,which means that they need to be captured and sequestered safely. [2] Although Xe is generated as fission product, by the time the fuel is reprocessed, all the radioactive isotopes of Xe have decayed to very low concentrations. Radioactive 85 Kr has al ong half-life (t 1/2 = 10.8 years) and therefore must be captured and removed to prevent its uncontrolled release into the atmosphere.Further presence of any trace amounts of radioactive Kr in Xe rich stream as ab yproduct is not suitable for practical use.T hese factors incentivize the development of an alternative technology for aless energy-intensive,more cost-effective,and safer process to capture Kr and Xe from reprocessing operations.I nt his regard, Zeolites and activated carbon have been evaluated for Xe/Kr separation at ambient conditions,h owever their low Xe selectivity and adsorption capacity renders them impractical. [3] Metal-organic materials (MOMs) [4] have also been widely investigated in terms of this gas separation due to their promising features including their inherent structural modularity,e xtra-large surface area, good thermal stability and recyclability. [5] However, MOMs that rely on unsaturated metal centers have many drawbacks,s uch as their strong tendency to interact with water and the high energy costs associated with their activati...