In this study, a mixture of N-methyl-2-pyrrolidinone (NMP) and ionic liquid (IL) was proposed as the entrainer for the separation of benzene and thiophene by extractive distillation. [EMIM](+)[BF4](-) might be a suitable IL for investigating the selectivity and capacity together using the COSMO-RS model. The experimental vapor-liquid equilibrium (VLE) results indicated that the addition of IL did not obviously improve the relative volatility of benzene to thiophene; however, the content of NMP in the vapor phase could be significantly reduced using the combination of NMP and [EMIM](+)[BE4](-) in comparison to the benchmark solvent NMP. Moreover, the UNIFAC model was extended and the corresponding interaction parameters were obtained by correlating the ternary (benzene + thiophene + NMP) and quaternary (benzene + thiophene + NMP + [EMIM](+)[BF4](-)) VLE equilibrium data obtained in this work at ambient pressure. (C) 2015 Elsevier B.V. All rights reserved.