This paper is a continuation of our investigation into the determination of activity coefficients at infinite dilution (gamma(13)(infinity)) of hydrocarbons in polar solvents by medium pressure gas liquid chromatography (g.l.c.). In this work the activity coefficients at infinite dilution for a series of alkanes (C-6-C-8), 1-alkenes (C-6-C-8), 1-alkynes (C-6-C-8), and cycloalkanes (C-6-C-8) have been measured in the industrially important polar solvents N-methyl-2-pyrrolidinone, or dimethyl sulphoxide, or N,N-dimethylformamide at T = 283.15 K and T = 298.25 K. The activity coefficients of some of the solutes discussed in this work would be difficult to determine by any other method because of their low solubility in polar solvents. The techniques used here have been described previously by the authors. The mixed second virial coefficients used in this work were determined assuming the principle of corresponding states, the Hudson and McCoubrey combining rules for T-12(c), the Lorentz rule for V-12(c), and the McGlashan-Potter equation.