Separation of aliphatic and aromatic compounds was studied using the high molecular weight trihexyl-tetradecylphosphonium cation (thtd-Ph) based ionic liquids (ILs) with the anions chloride [Cl], tetrafluoroborate [BF4], and bis(trifluoromethane sulfonylimide)[(Tf)(2)N]. Infinite dilution activity coefficients (gamma(infinity)) for these novel solvents have been measured at T = (308.15, 318.15, and 328.15) K, using gas-liquid chromatography. The aliphatic solutes studied were normal alkanes (pentane, hexane, heptane, octane), alkenes (hexene, heptene, octene), alkynes (hexyne, heptyne, octyne), cycloalkanes (cyclopentane, cylcohexane, cycloheptane), and alcohols (methanol, ethanol). The aromatic solute studied was benzene. The molar excess enthalpy at infinite dilution (H-E,H-infinity) was calculated using the temperature dependency of gamma(infinity) data. The selectivity values for the phosphonium-based ILs indicate a poorer separation capability than imidazolium-or pyridinium-based ILs. The values of gamma(infinity) have been found to be in the following order: alkanes > alkenes > alkynes > aromatics. Selectivity values indicate the IL [thtd-Ph][BF4] as an effective solvent for the separation of benzene + alcohol systems. The gamma(infinity) measurements on the phosphonium ILs were further validated using the a priori COSMO-RS method. The COSMO-RS prediction was first benchmarked on the values obtained with [thtd-Ph] tris(pentafluoroethyl) trifluorophosphate, which gave the average absolute deviation (AAD) to be 15 %. Using COSMO-RS, gamma(infinity) values were obtained for the three phosphonium ILs, and the corresponding AADs were 9%, 8%, and 16%.