It is generally recognized that many cases of so-called diffusion in reality involve much more complicated processes than would be suggested by Fick's Law alone. The diffusion rate is not entirely controlled by the concentration gradient, but depends to a great extent on the solubility of the absorbate. That is, deviations from Fick's Law are to be expected when intimate interaction occurs between the absorbate and absorbent. Previous work in these laboratories has suggested that the transport of water and alcohol through keratin belong to this class of diffusion phenomena so that only the more general form of Fick's Law is applicable; i.e. the diffusion coefficient is a function of the concentration of the diffusing substance. In the present article the variation of the diffusion coefficient of water in horn keratin is investigated quantitatively, and a theory of sorption formulated by Cassie, is applied to the problem with considerable success.