In the sodium loops of a fast reactor, mass transfer occurs due to the interaction of flowing sodium on stainless steel surfaces. Under the non-isothermal conditions prevailing in the loop some elements are preferentially leached from the surface layers of the hot zone and transported by sodium to the cooled zone where deposition may take place. The available information on the mass transport in non-isothermal sodium loops has been summarised, and an attempt has been made to understand the mechanisms involved, of which the chemical reactions at the sodium-stainless steel interface are especially important. The rate of diffusion towards the solid/liquid interface may be the rate-determining step in some of these reactions. When a ferritic surface layer is formed by the selective removal of austenitic stabilizing elements, diffusion of alloying constituents through the ferritic layer limits the growth of this layer. Only when the surface film is adherent, the diffusion across this layer becomes important. NaCrO/sub 2/, for instance, has poor adherence, and a surface film of this compound may not inhibit further corrosion.