A new isotherm model, derived for a concave-down Scatchard curve, is presented to tit the experimental results for pepsin and chymosin adsorption to pepstatin A affinity membranes. The new model is converted into a kinetic expression and used for the fitting of association curves. The results from the new kinetic model shaw a better agreement with the experimental data than the Langmuir kinetic model. The new kinetic model is also used together with the affinity membrane model to predict the single-solute breakthrough curves, which performs well in matching the experimental curves.