The objective of this work was to develop a model that takes into account the unique free-volume effect of water. The free-volume percentage of water is less than that of most polymers. This is opposite to the free-volume behavior of most solvents with polymers. As a result, the free-volume term in the UNIFAC-FV model introduced by Oishi and Prausnitz fails for aqueous systems. In the new model a free-volume term derived from the generalized van der Waals partition function is added to the combinatorial and residual terms of UNIFAC. In UNIFAC the geometric parameters were generally obtained directly from the van der Waals values. For water, however, these parameters were regressed. With the new free-volume term better predictions were obtained when the standard van der Waals geometric parameters for water were used. The new model was compared with the UNIFAC-FV model of Oishi and Prausnitz and the Entropic-FV model of Elbro et al. The new model was found to give better predictions for the aqueous polymer systems as well as for the non-aqueous polymer systems. (c) 2004 Elsevier B.V. All rights reserved.