A regular solution model was used to describe the Gibbs free energies of mixing for the solid and the liquid of a binary solvent, from which the compositions of the equilibrium phases were calculated on the principle of equal chemical potentials of the components and a phase diagram was thereby constructed by plotting these compositions at different temperatures. The phase diagrams so calculated with the model parameters set to different values well described the experimental findings from the phase diagrams of binary carbonates previously measured. The modeling results showed that the carbonates were essentially insoluble in the solid state and miscible in the liquid state, the intermolecular forces between any pair of linear carbonates were nearly constant, and the attraction between a linear and a cyclic molecule was considerably weaker than the average of the linear-linear and cyclic-cyclic attractions. They also showed that a greater difference in the melting points of the components caused the eutectic point to be closer to the melting point of the low-melting component, a weaker intermolecular attraction between the component molecules caused the solubility curves to be more convex up, and these two effects were independent of each other. It was further suggested that the compatibility between two carbonate liquids was to a large extent determined by the difference in the number of molecular conformations allowed for by the rotatable single bonds in their molecules, which would also account for the changes in other properties of the carbonates.