The aim of the present study is to assess the potential of ionic liquids (ILs) for natural gas sweetening purposes. The solubility of gas mixtures containing carbon dioxide (CO2) and methane (CH4) has been measured in the following ILs: 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, 1-ethyl-3-methylimidazolium diethylphosphate, trihexyltetradecylphosphonium dicyanamide, and trihexyltetradecylphosphonium bis(2,4,4-trimethylpentyl)phosphinate. The experiments involve bubble-point measurements in the temperature range 303.15-363.15 K and at pressures up to 14 MPa using a visual synthetic method. The influence of the gas composition on the bubble-point pressure was investigated for three gas mixtures containing 25 mol% CO2-75 mol% CH4, 50 mol% CO2-50 mol% CH4 and 75 mol% CO2-25 mol% CH4. The Peng-Robinson (PR) equation of state (EoS) in combination with van der Waals mixing rules, using only binary interaction parameters, was applied to predict the ternary vapor-liquid equilibrium data and to extract the real CO2/CH4 selectivities (i.e., the selectivity of CO2 in the presence of CH4 in the ternary system CO2-CH4-IL). This real selectivity does not differ significantly from the ideal selectivity (i.e., the ratio of the pure gas Henry constants) even for mole fractions of IL as low as 0.7 and regardless of the gas phase composition. (C) 2014 Elsevier B.V. All rights reserved.