A statistical thermodynamic model is developed for the nonlinear multicomponent protein adsorption equilibrium on ion exchanger. This model takes into account the electrostatic interactions between the adsorbent surface and protein molecules, as well as the lateral interactions between adsorbed protein molecules. The model is, therefore, in accord with the nonstoichiometric nature of the electrostatic adsorption of protein. There are two categories of model parameters: one corresponds to the adsorption affinity of the protein and the other is descriptive of the interactions between the adsorbed molecules. So, all the model parameters have definite physical meanings, and for the adsorption equilibria of single protein, there are only two model parameters. By comparison with batch adsorption equilibrium data of bovine serum albumin, the model is found to fit the experimental data well. The effects of buffer type, pH and ionic strength on the model parameters are reasonably interpreted by the electrostatic and thermodynamic theories. (c) 2006 American Institute of Chemical Engineers.