ABSTRACT:In a previous study we modified a double lattice model by introducing a new interaction parameter, which improved the mathematical approximation defect, and gave a new expression for the Helmholtz energy of mixing. In the model the universal constants C  and C ␥ in the primary lattice were determined by comparing them with literature Monte Carlo simulation data, which is the only case for r 1 ϭ 1 and r 2 ϭ 100 (case I). In this study we introduce new universal constants, C  and C ␥ , as a function of the chain length of a polymer in a solvent (case II) by comparing them with other literature simulation data for various polymer chain lengths. The proposed model is compared with polymer-solvent systems. In an upper critical solution temperature phase behavior the theoretical results of case II were improved over those of case I. However, in a lower critical solution temperature phase behavior those of case I were not very sensitive to C  and C ␥ .