In this study, two predictive methods to estimate solvent free-volume parameters are proposed. In one method, solvent free-volume parameters can be estimated using the viscosity data predicted from a group contribution method. To test the validity of the method, solvent selfdiffusion coefficients in polystyrene are correlated and predicted, in conjunction with Vrentas and Duda free-volume theory, using two sets of solvent free-volume parameters determined from predicted and experimental viscosity data. For correlations, the results using both sets of solvent free-volume parameters are satisfactory. For predictions, however, the solvent free-volume parameters estimated from predicted viscosities represent the diffusion behavior better than the parameters obtained from experimental viscosity data at higher solvent concentrations, while the trend is reversed at lower solvent concentrations. In the other method, if free-volume parameters for some solvents in a series are known, the parameters for other solvents in the same series can be predicted. Using this method, the free-volume parameters of four n-alkane gases are estimated.