The orthobaric liquid-liquid phase diagrams for the five binary mixtures of propanenitrile with the n-alkanes C-5-C-8, and C-10, have been determined by visual observation of the separation temperatures of mixtures of known mass composition. The results have been fitted to a conventional modern expression incorporating where needed Wegner extended-scaling terms. Various aspects of coexistence curve symmetry have been explored, particularly symmetry of the critical composition, i.e., whether it falls close to 0.5, and symmetry of the diameter, i.e., whether it falls parallel to the temperature axis. Neither mole fraction nor volume fraction fully meets either requirement. A new variable tau based on a forced fit to tau(c),= 0.5 reveals both kinds of symmetry and the data for all five mixtures when expressed in terms of a reduced temperature T/T-c, as a function of tau fall on a universal curve. The implications of this for the coexistence curves for bihomologous mixtures are briefly discussed.