Because of its low thermal conductivity, krypton is a useful gas for the vapor space of double- (or triple-) pane windows. However krypton is more expensive than argon, currently used for most of multi-pane windows. The high price of krypton is due to the energy-intensive cryogenic process for its recovery from oxygen that is obtained from air. Ionic liquids may provide a cost-effective absorption process for separation of krypton from the oxygen stream of a liquid-air plant. The polarizability of krypton is higher than that of oxygen; therefore, krypton solubility may be significantly higher than that of oxygen in a suitable highly polar ionic liquid. Experimental solubility measurements are needed for solvent selection. We developed three methods to measure the solubilities of sparingly-soluble gases in selected ionic liquids in the temperature range 298 to 353 K. We have compared different ionic liquids in terms of their capacity for krypton and selectivity of krypton over oxygen. Based on data for more than 15 ionic liquids, ionic liquid trihexyltetradecylphosphonium bis (2,4,4-trimethylpentyl) phosphinate [3C6C14P][(C8H17)2PO2] gives the best results. [3C6C14P][(C8H17)2PO2] shows selectivity 3 for krypton over oxygen and capacity often three to four times higher than that of other ionic liquids studied in this project. In addition to krypton and oxygen, we have made solubility measurements for xenon, argon and nitrogen in [3C6C14P][(C8H17)2PO2]. This ionic liquid gives selectivity 4 for xenon over krypton. We present experimental solubilities for O2, Kr, Xe, N2, and Ar in [3C6C14P][(C8H17)2PO2]. The viscosity of [3C6C14P][(C8H17)2PO2] is high. Because moderate viscosity is important for an industrial process, we also present viscosity and density data for this ionic liquid. Lower viscosity can be achieved by raising the temperature or by adding small amount of low- molecular liquid. We present experimental data. for the effect of water and temperature on viscosity and density. To lower viscosity, we are now looking at some other ionic liquids that are chemically similar to [3C6C14P][(C8H17)2PO2] but have lower molecular weight. Finally, we present first thoughts about a possible process design.