A new model based on Eyring's absolute rate theory for the prediction of solvent activities in binary polymer solutions is presented. This model requires experimental viscosity and density data. The model results for various solutions of poly(ethylene glycol)s with molecular weights of 200, 300, and 6000 in water and for poly(propylene glycol) with a molecular weight of 2025 in ethanol are compared with experimental data at different temperatures and concentrations. There is good agreement between the calculated and experimental results, and the overall average mean relative deviation of the model is less than 1.2%. (c) 2005 Wiley Periodicals, Inc.