To understand the structure and morphology of the membrane in different concentrations of the polymer, it is necessary possessing enough knowledge about the homogeneous/nonhomogeneous regions. In the current work, the effects of some alkanes as additive to the PES / NMP Addetive / Water system including:n-hexane , n-decane and tetradecanes were studied. Then the binodal curves were calculated and plotted with considerable accuracy. The Flory–Huggins model was used to do the predictions. By calculating the interaction parameters as well as optimization and fitting, we were able to obtain the acceptable accuracy to predict the cloud points at the higher concentrations.