Proton transport phenomena across a composite membrane (PBM), where an anion exchange layer composed of poly( sulfone) modified with quaternary amino groups was pasted on a cation exchange layer composed of a sulfonated copolymer of poly(styrene) and poly(divinylbenzene), were investigated. This composite membrane shows the characteristics of a bipolar membrane based on current-voltage measurements. If the voltage is applied from the anion-exchange layer, water is dissociated into H+ and OH-, and then OH- is transported to the anode through the poly(sulfone) layer, which suggests the possibility of polymer decomposition of the anion-exchange layer by the base. For the separation of H+ and Al3+ from their mixed solution by the PBM, the voltage is always applied from the anion exchange layer to the cation exchange layer to cause water splitting. However, there is no evidence of poly(sulfone) decomposition during H+ and Al3+ separation though the reverse bias voltage is applied to this membrane. H+ is transported from the external solution and combined with dissociated OH- from the intermediate surface in the anion exchange layer to produce water. The dissociated H+ in the intermediate surface is transported to the cation exchange layer side. Therefore, the poly(sulfone) layer is always in a neutral state, which implies that PBM has resistance to the alkaline atmosphere.