Several commercial ion-exchange membranes (FumaTech, DuPont) were tested in the electrodialytic oncentration of NaOH solutions in the range 1-5 M. It was found that the FumaTech cation-exchange membrane showed better performance than the bilayer Nafion membranes (DuPont) because of lower diluting effect caused by the electro-osmotic flow. However, neither of the tested membranes showed high enough efficiency for a practical application. The experimental results were modeled using the extended Nernst-Planck and Donnan equations. It was found that the derived model with two fitted parameters (effective fixed-charge concentration, tortuosity-porosity factor) satisfactorily describes the investigated process in the concentration range 1-4 M.