Concentration polarization is studied using a commercial anion and cation exchange membrane. Current-voltage curves show the occurrence of an overlimiting current. The nature of this overlimiting current is investigated in more detail, especially with respect to the contribution of water dissociation, pH measurements reveal that water dissociation is more pronounced in case of the anion exchange membrane than with the cation exchange membrane when the limiting current is exceeded. However, even with the anion exchange membrane it is found that the contribution of water dissociation is very low and more than 97% of the current is carried by the salt ions. Furthermore measurements are described showing that the membrane permselectivity (co-ion transport) remains constant in the overlimiting region. This means that the overlimiting current is virtually all carried by the salt counter ions for the two membranes investigated.