A new phosphonium ionic liquid was investigated as a possible solvent for use in the separation of aromatic hydrocarbons from aliphatic hydrocarbons. The activity coefficients at infinite dilution, gamma(infinity)(13), for 14 solutes: n-alkanes, alk-1-enes, alk-1-ynes, cycloalkanes, and aromatic hydrocarbons in the ionic liquid trihexyltetradecylphosphonium-bis-(2,4,4-trimethylpentyl)-phosphinate [3C(6)C(14)P][(C8H17)(2)PO2] were consequently determined by gas-liquid chromatography at four temperatures T = (303.15, 308.15, 313.15, and 318.18) K. The partial molar excess enthalpies at infinite dilution values, Delta H-1(E,infinity), were calculated from experimental gamma(infinity)(13) values obtained over the temperature range studied. The selectivities for the n-hexane/benzene, hex-1-yne/benzene, and cyclohexane/benzene separations scenarios were calculated from the gamma(infinity)(13) and were compared to studies from the recent literature for results of other ionic liquids, as well as NMP and sulfolane, taken from the recent literature. The results indicate that [3C(6)C(14)P][(C8H17)(2)PO2] would make a poor solvent for the separation of an alkane or alk-1-ene or cycloalkene from benzene. The results do, however, expand our knowledge and point to some interesting facets of (solute + ionic liquid) interactions. (C) 2008 Elsevier Ltd. All rights reserved.