For liquid crystalline pure compounds exhibiting re-entrant phenomena the chemical potential difference between two successive phases along the isotherm is supposed to have simple pressure dependence. For binary mixtures of such compounds the thermodynamic approach called equal Gibbs energy analysis is applied to the study of their pressure-composition phase diagrams. Many types of isothermal phase diagrams which exhibit re-entrant phase behaviour can be predicted by assuming perfect or non-perfect nature (regular and non-zero excess molar volume) of the solutions. For the mesogenic system hexyloxy-octyloxy cyanobiphenyl, it is shown that the pressure-composition phase diagram is more successfully described with a regular solution hypothesis than with a perfect hypothesis.