Ionic liquids provide a possible absorption process to extract krypton from air. The feed for such a process is an oxygen stream from a liquid-air plant. An effective ionic liquid is [P(14)666][TMPP]; in that solvent, the solubilities of some pertinent common gases are appreciably larger than those in conventional ionic liquids, and the selectivity Kr/O-2 is close to 3. A nonvolatile ionic liquid is preferred over a hydrocarbon solvent because of safety and simpler solvent recovery. Because, the viscosity of [P(14)666][TMPP] is very high, 20 wt.% [BHMIM][AC] is added to reduce the viscosity by one order of magnitude without significantly reducing solvent capacity and selectivity. This work provides extensive fundamental data (solubility, density and viscosity) required for process design. (C) 2015 Elsevier B.V. All rights reserved.