Using the thermodynamic expression for the Soret coefficient for diluted mixtures, expressed through the temperature derivative of the molecule chemical potential at the constant pressure, statistical mechanics is applied to relate this expression to the microscopic molecular parameters. When the contribution of the internal degrees of freedom in molecules is accounted for in the calculations, the results are equivalent to previous approaches, adding new terms to the Soret coefficient. These terms are related to the differences in molecular vibration and rotation between the solute and solvent molecules. These "internal" contributions to molecular thermodiffusion explain the isotopic effect observed in molecular systems. The theory describes most of the experimental data on the thermodiffusion of isolated molecules placed in non-electrolyte liquids. The approach also reveals a strong dependence of molecular thermodiffusion on molecular symmetry.