Transport of H+ ions across a capillary system (a microporous polymeric membrane) is studied for different HCl concentrations. From electrokinetic measurements, information about membrane properties (resistance and transport number) and membrane/solution interactions (streaming or zeta potentials) are obtained. Results show a decrease of these parameters when concentration increases. The effect of mixed salts (HCl + NaCl) on concentration and zeta potential values is also studied. Changes in the membrane charge related to the pH of solutions are obtained.