In this paper, a mathematic model has been developed to simulate a modified two-phase electro-electrodialysis (MTPEED) process in which an oil/water (O/W) emulsion is used as the catholyte. Previous results have demonstrated that the process is highly efficient in the recovery of organic acids. The fundamental aspects of this model are expressed by the overall mass transport equation and the total voltage equation. The mass transport equation includes several parameters, such as the electro-migration coefficient, the solute diffusion coefficient, the electro-osmotic coefficient, the concentration-osmotic coefficient as well as the current density and membrane selectivity coefficient. The voltage equation includes the electrode potential, the membrane potential, the solution resistance and the membrane resistance. The extraction equilibrium and the conductivity of the O/W emulsion were experimentally determined and calibrated. The model can predict the lactic acid concentration and the solution volume versus operating time well. In the context of this model, we discuss the operating mode's effect on the MTPEED process and the quantitative effect of some design variables. (c) 2008 Elsevier B.V. All rights reserved.