Thermal diffusion or Soret effect is a phenomenon of mass transport in fluid mixtures driven by temperature gradients. In this paper, we have studied thermal diffusion coefficients for different binary n-alkane mixtures (equal mass fraction) through both theoretical and experimental methods. The theoretical model is based on the irreversible thermodynamics. Particularly, the nonequilibrium thermodynamic property tau is examined and remodeled with a consideration of vapor-liquid equilibrium in binary mixtures. The experimental method is based on the thermogravitational technique. The new model provides a good agreement with the experimental data from this study and in the literature.