Electrodeionization (EDI) is being applied more and more to produce ultrapure water, especially in the semiconductor industry. The continuous electrical regeneration of the ion-exchange (IEX) mixed bed is the main advantage of this recent technology. EDI couples two well known effects: electrodialysis and IEX. In spite of its rapid development, there are no established theories, design equations nor clear mechanisms of regeneration and transport. The present research work deals with EDI process efficiency. We have investigated the influence of the applied voltage V and the flow rate Q on cell efficiency by measuring current intensity I, and inlet C-in and outlet C-out concentrations of the treated solution. The efficiency R is defined by: R (%) = 100 (Delta C/C-in) Q/Q where C = C-in-C-out and Q Delta C = J is the mass transfer flux. The main finding was that an original, empirical, and simple equation between the efficiency R and the flow rate Q is established: R = K'Delta VQ(-n) where n approximate to 0.5 or LogR = Log (K'Delta V)-n Log (Q), a linear logarithmic equation between Rand Q. The mass transfer flux J = Q Delta C is then directly proportional to Q and inversely to root Q : J = KQ(n) approximate to KQ(0.5). This is an important result because it presents strong analogies with the habitual equations of electrochemical hydrodynamics (rotating, porous and packed-bed electrodes). The other interesting results are: (1) the current vs. voltage characteristic curve is constituted of three main linear parts, with an optimal zone (4.5-12 V); and (2) the water dissociation threshold voltage was clearly shown (12 V) and coincides with the beginning of a decrease in efficiency.