Activity coefficients for hydrocarbon solutes at infinite dilution in 1-methyl-3-octylimidazolium chloride have been measured using the medium pressure gas-liquid chromatography method. The hydrocarbon solutes used were n-pentane, n-hexane, n-heptane, n-octane, 1-hexene, 1-heptene, 1-octene, 1-hexyne, 1-heptyne, 1-octyne, cyclopentane, cyclohexane, cycloheptane, benzene, and toluene. Activity coefficients at infinite dilution were determined at the following three temperatures (298.15, 308.15, and 318.15) K. Selectivities for benzene and the hydrocarbons are presented and the results indicate that 1-methyl-3-octyl-imidazolium chloride is a reasonable solvent for the separation of an alkane or an alkene from benzene. (C) 2003 Elsevier Science Ltd. All rights reserved.