A steady-state model was established to simulate EDI process for producing ultrapure water (MixEDI), the dilute compartment of which is filled with mixed cation and anion-exchange resins By calculating the mathematical model which includes water dissociation mechanism, ionic status of ion-exchange resin, concentration polarization status and the concentration distribution of water dissociation products are obtained The influence of water dissociation on the current efficiency, removal rate and pH value of EDI effluent is investigated The existence of water dissociation catalyst at anion-exchange membrane (AM) makes the water dissociation current of the AM much larger than that of CM. The result is that the amount of electro-regenerated cation-exchange resins is much larger than that of anion resins. This is the reason why the removal rate of salt cation much larger than that of salt anion in EDI for producing ultrapure water Thus, at the target percentage removal, water dissociation at AM surface is excessive and the one at CM surface is insufficient We assume that there is also some water dissociation catalyst at CM surface It is found that the Improved water dissociation at CM could increase the percentage removal of salt anions and the current efficiency. (C) 2010 Elsevier Ltd All rights reserved.