Abstract 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, χ, 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°C higher than the experimental values. It was found that a small difference in the χ 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 χ was calculated from the best-fit values of X 12 and Q 12. The parameter χ calculated from theory showed a composition dependency which was in good agreement with those obtained from the neutron scattering measurements.