With the introduction of the Simha-Somcynsky（SS）equation-of-state（EOS）into the Vrentas-Duda model，a modified diffusion coefficient model based on the EOS was developed for polymer-solvent systems.In the modified model，the measurement of polymer viscoelasticity was not required，and the parameters related to the polymer could be determined solely by the characteristic parameters of temperature and volume in the SS EOS.Therefore，it has better predictive capability compared with the original model.The calculated diffusion coefficients of benzene，toluene，ethylbenzene and chloroform in polystyrene，polyisobutylene and poly（vinyl acetate）indicated that predictions of the modified model were consistent with the experimental results.