Vapor-liquid equilibria (VLE) have been measured for five 1-hexene/n-hexane/ionic liquid systems and 1-hexene/n-hexane/NMP (N-methyl-2-pyrrolidone) system with a headspace-gas chromatography (HSGC) apparatus at 333.15K. The ionic liquids investigated were 1,3-dimethylimidazolium tetrafluoroborate [C2MIM](+)[BF4](-), 1-butyl-3-methylimidazolium tetrafluoroborate [C4MIM](+)[BF4](-), 1-methyl-3octylimidazolium tetrafluoroborate [C8MIM](+)[BF4](-), 1,3-dimethylimidazolium dicyanamide [C(2)MlM](+)[N(CN)(2)](-) and 1-octylquinolinium bis(trifluoromethylsulfonyl)amide [C(8)Chin](+)[BTA](-). It was found that at low feeding concentration of 1-hexene and n-hexane, the separation ability of ionic liquids is in the order of [C2MIM](+)[BF4](-) > [C4MIM](+)[BF4](-) approximate to [C2MIM](+)[N(CN)(2)](-) > [C8MIM](+)[BF4](-) > [C(8)Chin](+)[BTA](-), which is consistent with the priori prediction of the COSMO-RS (conductor-like screening model for real solvents) model. But at high feeding concentration, the separation ability of ionic liquids is in the order of [C2MIM](+) [BF4](-) < [C4MIM](+)[BF4](-) approximate to [C2MIM](+) [N(CN)(2)](-) < [C8MIM](+) [BF4]< [C(8)Chin](+) [BTA](-). The liquid demixing effect should be taken into account. The activity coefficients of 1-hexene and n-hexane at infinite dilution calculated with the COSMO-RS model were correlated using the NRTL, Wilson and UNIQUAC model. In this work the predictive results from the COSMO-RS model and UNIFAC model for the 1-hexene/n-hexane and 1-hexene/n-hexane/NMP systems were compared. The UNIFAC model is one of the most important academic contributions by Prof. Jurgen Gmehling. (c) 2006 Elsevier B.V. All rights reserved.