The physical significance of concentration-dependent diffusion coefficients is examined. It is shown that such concentration dependences cogently follow from typical sorption isotherms. As an example, NMR images of the progress of isovalent competitive ion exchange processes in alginate gels are compared with the predictions of diffusion models assuming Henry or Langmuir sorption isotherms. It is shown that the model on the basis of the Langmuir isotherm provides a much better correspondence with the experimental data than the linear absorption model based on the Henry isotherm. The better correspondence of the Langmuir model is due to the fact that saturation effects are taken into account in this model, which is based on the law of mass action.