A semiempirical equation of Gibbs free energy of mixing DeltaG for the system polystyrene in methyleyclohexane has been determined based on the Flory-Huggins equation by using a series of coexistence curve data. The functional form of the interaction parameter g with respect to chain length of the polymer was determined so as to express the critical universality. The resultant equation of DeltaG was used to prepare microcapsules of controlled size distribution by means of coacervation in the spinodal region.