  We study thermal diffusion in binary mixtures in the framework of
non-equilibrium thermodynamics. Our formal result displays the role of partial
enthalpies and Onsager's generalized mobilities. The mobility ratio provides a
measure for the irreversible character of thermal diffusion. Comparison with
experimental data on benzene, cyclohexane, toluene and alkanes shows that
irreversibility is essential for thermal diffusion, and in particular for the
isotope effect.

Soft Condensed Matter