This paper investigates the behavior of bovine serum albumin (BSA) during water dissociation on a bipolar membrane (BPM). BSA-modified BPM is prepared by immersing polyethylene anion exchange membrane in different concentration solutions of BSA, then casting the solution of sulfonated poly(phenylene oxide) (SPPO) in dimethyl formamide. The modification of BSA was evidenced by atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The junction thickness was evaluated by electrochemical impedance spectroscopy (EIS). The results showed that the typical I-V curves for bipolar membranes were heavily affected by the BSA modifications: the more the adsorbed amount of BSA, the larger the potential drop across a bipolar membrane. The new phenomena is underlined by the intrinsic properties of BSA molecules: steric effects give rise to an increase in the thickness of the depletion layer, amphoteric properties weaken the electric field of the junction, and hydrophobicity makes the junction less wet. All of these cause negative effects on water dissociation on a bipolar membrane. (C) 2004 Elsevier Inc. All rights reserved.