The tie-line composition of three quaternary system namely 1-ethyl-3-methylimidazolium acetate ([EMIM][OAc]) ([EMIM][OAc]) (1)+thiophene (2)+pyridine (3)+toluene (4), 1-ethyl-3-methylimidazolium ethylsulphate ([EMIM][EtSO4]) (1)+thiophene (2)+pyridine (3)+toluene (4), 1-ethyl-3-methylimidazolium methylsulphonate ([EMIM][MeSO3]) (1)+thiophene (2)+pyridine (3)+toluene (4) were experimentally determined at 298.15 K. The measured tie-line data were successfully correlated with the nonrandom two liquid and UNIversal QUAsiChemical model prediction which gave less than 1% root mean square deviation (RMSD). [EMIM][MeSO3] looks to be a promising solvent for the simultaneous separation having distribution ratios less than unity for both thiophene and pyridine. The quantum chemical-based conductor like screening model for real solvent (COSMO-RS) model was then used to predict the tie-line composition of quaternary systems. COSMO-RS gave the RMSD for the studied systems to be 8.41, 8.74, and 6.53% for the ionic liquids, respectively. (c) 2013 American Institute of Chemical Engineers AIChE J, 59: 4806-4815, 2013