There would appear to be at least two stages in the formation of fatty spue on leather: (1) the migration of fat from the centre to the grain surface, and (2) the crystallisation of the fat on the surface in the form of a large number of minute crystals. It is quite possible that progressive crystallisation of fats on the surface may, in itself, provide part of the motive force causing the fat to migrate from the interior. In the work described below, the presence of dyes, mordants, and detergents is shown to have a marked influence on the formation of fatty spue, as well as such physical factors as the thermal history of the leather, the presence of crystallisation promoters (seed crystals), and mechanical action. Whether the dyes, mordants, and detergents act merely through their effect on crystal growth, or whether they set up a concentration gradient from the centre to the surface cannot be decided from the evidence available. Oleic acid cannot be used successfully as a crystallisation inhibitor.