A new calculation procedure for free-volume parameters is considered in this work by using viscosity prediction methods and the Levenberg-Marquardt calculation scheme. All parameters used in the Vrentas-Duda free-volume theory can be estimated from pure component properties. The prediction results are compared with experimental data for some polymer/solvent systems. The diffusion coefficient calculated by Vrentas-Duda theory can be used in the modeling of membrane separation processes. (c) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 110: 3544-3-551, 2008