Abstract: On the basis of a thermodynamic model of liquid mixtures, the expression of excess Gibbs free energy derived for non - electrolyte solutions is more strict than those of Scatchard - Hildebrand theory of regular solutions. A new solubility parameter thereby defined equals to the cohesive energy density of the liquid divided by the square root of internal pressure. The prediction results of excess Gibbs free energy show that its deviation from the experimental values is less than those of SH theory and the modified SHFH theory for most of thirty - five binary liquid mixtures, especially in solutions of fluorocarbons and hydrocarbons where SH and SHFH theory are not applicable.