The specific retention volumes, V g , of all of the C 6–C 8 branched-chain alkanes were measured on a 20% SF-96 column at 80°, 100° and 120°, and the vapour pressures, p 0, at the same temperatures were calculated by three methods. A better agreement of the Antoine-type equation was found. Narrow grid plots of log V g as a function of log p 0 showed a fine structure depending on molecular branching, and a positive curvature owing to the influence of the activity coefficient, γ. The values of γ, which indicate the effect of the solute-solvent interactions on the gas chromatographic behaviour of the compounds, were calculated and correlated with the molecular structure and with some physical properties (molecular volume, density, surface tension and latent heat of vaporization).