The static solution head in an entrance desalting duct He, that in an exit desalting duct H-ex and their difference DeltaH were measured by electrodialysis of seawater using a practical scale electrodialyzer. Applying hydrodynamics to this electrodialysis system on the basis of the assumption that the flow pattern in a desalting cell is laminar flow, which was demonstrated from the relationship between the Reynolds number and the static head drop in a desalting cell, we evaluated: (1) the static head difference between an entrance and an exit of a current-passing section DeltaH(c), and that of a passageway DeltaH(w); (2) the spacer friction factor f(s), expressing the effect of a spacer to friction resistance; (3) the distribution coefficient of solutions flowing into every desalting cell theta; (4) the static head H-s,H-velocity head H-v and friction head H-f of a solution flowing through a duct; (5) the velocity head of solutions flowing into or out of current-passing sections through passageways H-v,H-w. The solution flow in a stack is classified as a one-way flow system, in which the flow direction in an entrance duct is identical with that in an exit duct, or a two-way flow system, in which the flow direction in an entrance duct and that in an exit duct are opposite to each other. In order to operate an electrodialyzer stably by maintaining a uniform DeltaH in a stack, it is preferable to adopt one-way flow system rather than two-way flow system, and further it is desirable to decrease the difference of H-v,H-w, between the values in an entrance passageway and an exit passageway, (H-v,H-w)(en) - (H-v,H-w)(ex). Solution leakage was evaluated by means of seawater electrodialysis based on the extended overall mass transport equation in which the volume flux is expressed by the terms of electro-osmosis, concentration-osmosis and leakage of solutions. The overall mass transport equation was introduced from the non-equilibrium thermodynamics. We found that the solution leakage moving from a desalting cell to a concentrating cell and that moving from a concentrating cell to a desalting cell arise at the same time in the electrodialyzer. These phenomena are attributed to the fact that the pressure in a desalting cell becomes larger than that in a concentrating cell in some part, and that in a desalting cell becomes smaller than that in a concentrating cell in another part. These events arise because the distribution of solution velocity and friction head is not uniform in desalting cells. (C) 2004 Elsevier B.V. All rights reserved.