Using a simple activity coefficient model in conjunction with the Vrentas-Duda free-volume (FV) theory, the diffusion behavior of small molecular solvents between and amongst polymer chains is then predicted for several polymer/solvent systems over a wide range of concentrations. The estimations are comparable with experimental data for some systems. For diffusion over- and under-predicted systems, a pre-exponential factor D-0 can be adjusted to improve the predictions. It is found that the entropic-FV model is a suitable alternative to the Flory-Huggins equation for predicting mutual-diffusion coefficients in polymeric solutions. To understand the impact of the model on the prediction thoroughly a sensitivity analysis is done for free-volume. The influence of the free-volume of solvent is the opposite of that of the polymer.