The interaction energy parameter (X(12)), and the interaction entropy parameter (Q(12)) of the Flory’s equation-of-state theory were determined from the Flory-Huggins interaction parameter, chi, which was measured from scattering data for the miscible blend systems, polystyrene (PS)/poly(vinyl methyl ether) (PVME) and deuterated polystyrene (dPS)/PVME blends. The phase diagram of the polymer mixture was predicted from the values of X(12) and Q(12). The spinodal curve predicted from theory showed a similar shape to the experimental one although the lower critical solution temperature from theory is ca. 10-70 degrees C higher than the experimental values. It was found that a small difference in the chi values leads to a large shift of spinodal curve. On the other hand, the parameters X(12) and Q(12) were determined by fitting the EOS theory to experimental data, and then the parameter chi was calculated from the best-fit values of X(12) and Q(12). The parameter chi calculated from theory showed a composition dependency which was in good agreement with those obtained from the neutron scattering measurements.