The Perturbed Chain Statistical Association Fluid Theory (PC-SAFT) is used to investigate the solubility of carbon dioxide (CO2) and hydrogen sulfide (H2S) in several methylimidazolium bis (tri-fluoromethylsulfonyl) imide ionic liquids (ILs) or [C(n)mim][NTf2] where n=2, 4, 6, and 8. The pure component parameters of the ILs are estimated by fitting to experimental density data and binary solubility data of acid gases in ILs reported in literature. Two strategies are examined to model the ILs. In the first strategy, the ILs are treated as neutral molecules. In the second strategy, the ILs are modelled as two charged ions: imidazolium cation [C(n)mim](+) and bis (trifluoromethylsulfonyl) imide anion [NTf2](-). For each strategy, four different self association schemes are examined: non-associating, 2-site, 3-site, and 4-site schemes. The inclusion of self-association of the IL improves the calculated acid gas solubility. The 4-site association scheme with two donors and two acceptors provided the best results for almost all the investigated acid gases-IL binary systems, with an AARD of 2.76%-6.62% for H2S-ILs systems and 1.54% -4.98% for CO2-IL systems. Using these parameters, the solubility of ternary systems of CO2 and H2S in C(8)mimNTf(2) IL is successfully represented, with an AARD of 6.24% for CO2 and 7.99% for H2S, without the need for binary interaction parameters. The high pressure density of ILs and the binary solubility of CO2-Ws at high pressure is also represented with reasonable accuracy. The inclusion of the electrolyte term in the second strategy improves the high pressure density and solubility results as well as the predictive capability of the model by allowing for the examination of the effect of using different cation-anion combinations. (C) 2017 Elsevier B.V. All rights reserved.