Plasticization of glassy polymers by small molecules was approached by the "concentration-temperature superposition" principle. The major effect of the plasticization by small molecules is on the reduction of the glass transition temperature. The present study suggests that the dependence of diffusion coefficients of small molecules on the penetrant concentration can be affected by the reduction of the glass transition of the penetrant/polymer system caused by a plasticization effect of the penetrant. With a WLF type shift factor, the concentration-dependent diffusion coefficient can be predicted. It is found that the calculated diffusion coefficient correlates very well to the experimental data. This study also proposes a prediction of time-lag values from the solubility and permeability measurements. Moreover, the diffusion coefficients for gases in glassy polymers can also be predicted using the time-lag values alone. The proposed diffusion model represents satisfactorily experimental data reported in the literature for CO2 in poly(ethylene terephthalate), in polycarbonate, and in polyacrylate. The prediction of diffusion coefficient in glassy polymers is seen to agree well with the experimental data, whereas the prediction of time-lag values is less accurate due to the error propagation in solubility measurement, permeability measurement, and time-lag measurement, respectively.