In this paper, a mathematic model of a three-compartment membrane electrodialysis in H2SO4-(NH4)(2)SO4-NH3 center dot H2O system has been developed to calculate the exit concentration of ammonium sulfate in the desalting cell. The model is established on the basis of electromigration and concentration diffusion, and it includes several parameters such as electrolyte solution diffusion coefficient, diffusion boundary layer thickness, current density as well as ionic transference number in the solution and in the membrane. These parameters can be deduced from a reliable procedure based on some experimental data. The model can predict the ammonium sulfate exit concentration according to actual operating conditions. In the context of this model, we discuss the current density effect on the exit concentration. The results showed that calculation data were in accordance with experimental data in large current density range, but deviated from them in a small current density range. The good agreement between experimental data and model data confirms the reliability of the model predictions. (C) 2009 Elsevier B.V. All rights reserved.