A new simple activity coefficient model for the prediction of solvent activities in polymer solutions is presented. The model consists of three terms, the combinatorial and the residual terms, which are taken from UNIFAC model, and a free-volume term derived from the van der Waals partition function with the mixed volume, the mixed free-volume and the mixed c-value being supposed to be given by linear mixing rules. Twenty-one polymer solution systems were used to test the applicability of the present model. Compared with the UNIFAC-FV Entropic-FV and Entropic-FV-1.2 models, it is found that the model proposed in this work can produce better results than those from the other three models. Furthermore, the model is valid for a broad range of polymer concentration and a wide range of temperature. (c) 2005 Wiley Periodicals, Inc.