A new method to determine the nonsolvent-polymer interaction parameter chi(13) in the casting solutions was developed. With the measured intrinsic viscosities of the polymer in the mixed solvents with different concentrations, the nonsolvent-polymer interaction parameter chi(13) could be calculated by the application of the formalism proposed by Campos et al. The values of chi(13) in membrane-forming systems of nonsolvent/N-methyl-2-pyrrolidone (NMP)/polyethersulfone (PES) were obtained by the proposed method. The nonsolvents used were water, I-butanol and diethylene glycol (DegOH). The stronger the nonsolvent, the higher the value of chi(13). In addition, the concentration of the nonsolvent at the binodal curve increased with decreasing chi(13) The binodal curves for these membrane-forming systems predicted from the Flory-Huggins theory with chi(13) Obtained by the proposed method were consistent with measured cloud point curves, which confirmed that the method proposed here was reliable.