Abstract: A mathematical model is proposed for predicting small molecule solvent diffusion coefficients in amorphous glassy polymers, based on free volume theory that is suitable for rubbery polymers, and with the consideration of the plasticization effect induced by the penetrant. Because this model uses the identical conceptual framework in which free volume theory is derived for solvent diffusion in rubbery polymers, parameters used in free volume theory remain valid for this work. Only one parameter is introduced to describe the plasticization effect, and it can be easily estimated by thermodynamic theory about the glass transition temperature depression. Therefore, no adjustable parameters appear in this expression, resulting in its usefulness in prediction. Comparison of the diffusion coefficients calculated by this new model with the present experimental data, including benzene, toluene, ethylbenzene in polystyrene (PS) and poly(methyl methacrylate) (PMMA) are performed, calculation results in this study are in good agreement with experimental data, for the correct estimation of the hole-free volume change above and below the glass transition.