We have developed a new Flory-Huggins model by adding a specific interaction parameter derived from a modified double-lattice model for the Helmholtz energy of mixing for binary liquid mixtures. This model is very simple and could be easily integrated into engineering applications. Using this revised model, we can successfully describe the phase behavior of polymer solutions with an upper critical solution temperature (UCST), a lower critical solution temperature (LCST), both UCST and LCST, and a closed miscibility loop. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 162167, 2010