The current-voltage curves of a composite bipolar membrane (CBM) were measured experimentally for the various mole fractions of formic acid + water, acetic acid + water and propionic acid + water mixed solution systems. The experimental results showed that a CBM has the characteristics of a bipolar membrane. The current-voltage characteristics were analyzed using Mafe's and Ramirez's ion transport theory by applying the chemical reaction model in the intermediate region, and the theoretical calculations could well explain the experimental results. The increase in the mole fraction of the organic acid causes the increase of the ion concentration in the solution, and this effect enhances the water splitting. On the other hand, the increase in the mole fraction of the organic acid causes the decrease of the water concentration in the space charge region of the membrane also, and the effect lessens the water splitting. Based on the above two effects, it could be expected that with an increase in the mole fraction of the organic acid, the maximum local effective value of a CBM resistance decreases firstly, and then increases when the mole fraction of water becomes rather low. The results that we found in our experiments correspond to the above discussion.