The new UNIFAC-vdW-FV model with the recently developed van der Waals free-volume term was found to give the best predictions for aqueous polymer systems as well as the non-aqueous polymer systems. The behavior of the new van der Waals free-volume term in organic solvents-water solutions has been studied. The application of the UNIFAC-vdW-FV model to the important water-PEO systems by modeling the effect of polymer molecular weight on water activity has been studied. Liquid-liquid equilibrium applications are discussed. The application of the new van der Waals free-volume term in modeling liquid-liquid equilibria with the UNIFAC model with the present interaction is discussed. The same liquid-liquid equilibria cases have been modeled using the UNIFAC-vdW-FV model with new group definitions and new interaction parameters. The liquid-liquid equilibrium results indicate that this approach is useful for emulsion polymerization processes.