Activity coefficients at infinite dilution, gamma(infinity)(i), have been determined for organic solutes in the phosphonium-based ionic liquid, tributylmethylphosphonium methylsulphate at T = 308.15 K, 318.15 K and 328.15 K. The experimental activity coefficient data presented are the first to be reported for this ionic liquid, through the use of the technique of steady-state gas-liquid chromatography. The organic solutes that were investigated were normal alkanes (pentane, hexane, heptane and octane), alkenes (1-hexene, 1-heptene and 1-octene), alkynes (1-hexyne, 1-heptyne and 1-octyne), cycloalkanes (cyclopentane, cyclohexane and cycloheptane) and benzene. The partial molar excess enthalpies at infinite dilution, H-i(E,infinity) were also determined for the solutes from the temperature dependency of the gamma(infinity)(i) values. The gamma(infinity)(i) data obtained for tributylmethylphosphonium methylsulphate in this investigation have been compared to those obtained for other ionic liquids available in literature; most notably that of trihexyl(tetradecyl)-phosphonium tris(pentafluoroethyl) trifluorophosphate. This comparison allows for an assessment of the potential for the use of different ionic liquids as solvents in industrial solvent-enhanced separation processes such as extractive distillation. (c) 2006 Elsevier B.V. All rights reserved.