The membrane potential of the composite membrane, TiO2/Al2O3. and the Al2O3 support were measured to characterize the membrane charge of TiO2/Al2O3 membrane and to make clear the effect of Al2O3 support As the results, it is shown that it is impossible to neglect the effect of the support due to its charge. It is also shown that the TiO2/Al2O3 membrane will be bipolar between pH 6.2 and 7.4.