- A thin convection-free cell was constructed for the characterization of cation-exchange membranes, with CuSO4 solutions confined between thick copper electrodes. Current—voltage curves were recorded for several homogeneous membranes. The limiting current, i.e. the flat portion of the curve, was 1.5–4 times lower than the theoretical prediction. At high voltage the current increases approximately linearly above its limiting value. Onset of noise and noise frequency were determined. It was suggested that the ion conductance of the membrane surface is not uniform. This inhomogeneity decreases the available area and, at high voltage, gives rise to local mixing of the unstirred layer by electroconvection. High limiting current was observed with a composite membrane comprising a porous heterogeneous layer and a highly permselective dense homogeneous layer (sulfonated polysulfone). This may be due to electroconvection in the porous part of the layered structure. The drastic difference of polarization at homogeneous sulfonated polysulfone membranes and at the composite membrane was confirmed in a laboratory electrodialysis stack.