In the present study a new mathematical model's outcome based on experimental data is considered to determine the diffusion coefficients in polystyrene/solvent systems as a function of solvent concentration. We used a calibrated Fourier transform infrared attenuated total reflectance (FTIR-ATR) instrument to collect the spectra from a thin layer of polymer solution covered optically dense ZnSe crystal. The collected spectra were transferred to the components' concentration, using principal component regression analysis, representing the compositional change of the polymer solution during the time. Two approaches were employed to obtain the diffusion coefficients of seven solvents in polystyrene solutions: first, we considered whole range of polymer concentration to obtain the diffusion coefficient by fitting the diffusion model on experimental data. In the second approach only early stage of evaporation process was figured, considering vitrification phenomenon in the upper layers of polymer solution film. For all solvents, higher values of diffusion coefficients obtained using second approach, showing a satisfying agreement with the literature. As a concluding remark, vitrification is an important event taken place during mass transfer processes in which it should be considered to find a reliable value for diffusion coefficient.