The current-voltage curves of a composite bipolar membrane (CBM) were experimentally measured by inserting the thin poly(acrylonitrile) (PAN) membrane between cation- and anion-exchange membranes for water and methanol solutions. In each solution system, 0.05 mol/L LiCl was used as the electrolyte. The measured results show that the thin PAN membranes enhanced the water- and methanol-splitting effect. This phenomenon can be explained by the protonation-deprotonation reactions occurring between the functional group of PAN (-C drop N, cyano) and the water or methanol molecules in the intermediate region of the CBM. The effect of niobium alloy (Nb3Ga), fullerene (C-60) and titanium oxide (TiO2) existing in the intermediate region of the CBM was also experimentally examined in this study. It was found that the effect of these compounds on water or methanol-splitting was not obvious. (C) 2000 John Wiley & Sons, Inc.