Tetraalkylphosphonium bis(2,4,4-trimethylpentyl)phosphinates show large solubilities for methane, ethane, ethylene, and propane. In these ionic liquids, solubilities of ethane are larger than those of ethylene. Therefore, these ionic liquids may be useful solvents for separation of ethane and ethylene; because the vapor pressure of ethylene is higher than that of ethane, the relative volatility ethylene/ethane is enhanced. However, the viscosities of these ionic liquids are too high for an industrial process. Low-viscosity 1-butyl-3-H-imidazolium acetate([BHMIM][AC]) is a suitable diluent for reducing the large viscosities of trihexyl tetradecylphosphonium bis(2,4,4-trimethylpentyl) phosphinate ([P(14)666][TMPP]) and tetrabutylphosphonium bis(2,4,4-trimethylpentyl) phosphinate ([P4444][TMPP]). Addition of 20 wt % [BHMIM][AC] gives a dramatic drop in the viscosities of these ionic liquids. Mixtures of [P(14)666][TMPP] or [P4444][TMPP] with 20 or 50 wt % [BHMIM][AC] show high solubilities for the four solutes when compared with those in other ionic liquids. In these mixtures, the solubility for ethane is higher than that for ethylene. (c) 2014 American Institute of Chemical Engineers