Vapour-liquid phase equilibria at 313.15 K for the four mixtures of propanenitrile with n-pentane, n-hexane, n-heptane and n-octane have been determined by the measurement of the total vapour pressure in a static apparatus. All mixtures display azeotropy, with the alkane mole fraction of the azeotrope moving from 0.938 in the C-5 mixture to 0.239 for the C-8 mixture. The excess Gibbs functions calculated by the Barker method (Barker, 1953) are positive, large, symmetrical and, within the range studied, relatively independent of the alkane chain length; their value increases from 1331 J mol(-1) for the C-5 mixture to 1442 J mol(-1) for the C-8 mixture, a pattern consistent with what is already known about such mixtures. The results are tolerably well described by the Weimer-Prausnitz modification of the Scott-Hildebrand solubility-parameter version of the regular solution theory, with a Flory-Huggins entropy of mixing that gave a good account of the upper critical solution temperatures of these and other members of this series of mixtures.