Following our previous work, we developed a new lattice model for systems containing two solvents and a polymer chain. The chain length of solvent molecules is taken as 1, while that of the polymer is treated as r in the model. Comparison with Monte Carlo simulation data shows the great advantages of this model over the traditional Flory-Huggins theory and the revised Freed theory especially in the critical region. When applied to real systems, it is found that this model also exhibits notable practical value for the engineering application. (C) 2007 Elsevier B.V. All rights reserved.