The concentration-dependent diffusion coefficient of fructose in precipitated chitosan membranes was determined using diffusion cells. Experiments showed that boundary layers on both sides of the membrane could not be neglected when the diffusion coefficient in chitosan is close to the free diffusion coefficient. Hence, the influence of the boundary layers was covered by measuring membranes with different thickness. The diffusion coefficient in chitosan at infinite dilution was found to be around the free diffusion coefficient. With increasing concentration, the diffusion coefficient in chitosan decreases faster than the free diffusion coefficient. Thus, the product of diffusion coefficient and concentration difference shows a maximum implying an optimum concentration for mass transfer.