- The voltage against current curves of cation exchange membranes have been studied. They have a characteristic shape with a region of slow current variation (the plateau) followed by a region of accelerated current growth as the voltage is increased (the inflexion). For currents much greater than the “limiting” one determined by the value at the “plateau” of the (V, C) curve, the current is transferred across the membrane mainly by the cations of the salt; the contribution of “water splitting” is minor. Experiments on suppressing convection near the membrane with the aid of microporous filters have shown that such convection cannot be responsible for the inflexion of the voltage against current curves, and for the passage through the membrane of current much greater than the “limiting” value. It was shown on the basis of a simple theoretical model that formation near the membrane of a region of the volume charge, reflected in replacing the electroneutrality condition by the full Poisson equation, may lead to passage through the membrane of currents greater than the “limiting” one; the voltage against current curves of the general form described above with an inflexion, were then obtained using computation. The asymptotic behaviour of these voltage against current curves at large voltages was studied.