The generalized lattice-fluid (GLF) model is extended to predict phase behaviors of polymer/solvent systems. The GLF model gives some difficulties in describing liquid-liquid equilibria (LLE) of binary polymer solution systems due to general assumptions on its derivation. An extended lattice-fluid (ELF) model is proposed by introducing a new universal constant (C-0) and a model parameter (kappa(11)). The proposed model is then compared with experimental data for polymer/solvent systems and polymer1/polymer2 systems, which exhibit lower critical solution temperature (LCST) behaviors. Theoretical predictions and experimental results are in good agreement.