Free-volume parameters for various solvents are estimated using viscosity-temperature data. Using these parameters in conjunction with the Vrentas-Duda free-volume theory, diffusion behavior is then predicted for several polymer/solvent systems over wide ranges of concentration and temperature. The prediction results are comparable with experimental data for some polymer/solvent systems. For other systems, however, the theory overpredicts diffusion behavior. To understand the discrepancy between the experiment and prediction, a sensitivity analysis is shown for each parameter in the theory. The predictions can be improved by modifying one of the parameters.