The activity coefficients at infinite dilution, gamma(13)(infinity), for hydrocarbons and methanol solutes in the ionic liquid 1-butyl-3-methylimidazolium octyl sulfate, [BMIM] [OcOSO(3)], have been determined by gas-liquid chromatography at temperatures of 298.15 K, 313.15 K, and 328.15 K. This work is part of our research focus on activity coefficients at infinite dilution, gamma(13)(infinity), in ionic liquids. The selectivity values have been calculated at T = 298.15 K, and the results indicate that the ionic liquid [BMIM] [OsOSO3] should not be considered to be a solvent for the separation of alkanes and aromatics. The partial molar excess enthalpy values at infinite dilution Delta H-1(E infinity) have also been determined at temperature 298.15 K and have been discussed in terms of intermolecular interactions and packing effects. The results have been discussed in terms of measurements of gamma(13)(infinity) for other ionic liquids taken from the recent literature.