Fifty years ago Chapman (1917) found in the course of his work on the general kinetic theory of gases the phenomenon of thermal diffusion (earlier discovered in a special case by Enskog 1911); this is the relative motion of the components of a mixture arising from a temperature gradient. There has been much experimental study of thermal diffusion since the first experimental confirmation (Chapman & Dootson 1917). in the terrestrial atmosphere where the principal constituents of air are molecular nitrogen and oxygen the thermal‐diffusion flux is practically negligible compared with the diffusive flux due to the pressure gradient. Thermal diffusion is, however, important in the thermosphere where helium and hydrogen diffuse through a region with high temperature gradients.