The separation of hexene isomers is very important but difficult since their close boiling points. Room temperature ionic liquids are green solvents and have potential for the separation of hexene isomers by extraction or extractive distillation. Therefore, isothermal vapor-liquid equilibrium data of 1-hexene and trans-3-hexene with 1-methyl-3-butylimidazolium hexafluorophosphate ([BMIM][PF6]) or with a complex of 1-isobutenyl-3-methylimidazolium tetrafluoroborate ([iBeM1M][BF4]) ionic liquids with AgBF4 were measured by a headspace gas chromatography method. The solubilities of I-hexene and trans-3-hexene in ionic liquids were determined experimentally. The NRTL model was used to correlate the binary vapor-liquid equilibrium data to obtain interaction parameters. NRTL predictions for the ternary system are compared with the experimental data. (c) 2006 Elsevier B.V. All rights reserved.