The chemical stability of a commercial bipolar membrane (Neosepta BP-1, Tokuyama Co.) was examined after immersing it in concentrated NaOH solution at different temperatures, for various periods of time. The physical and electrochemical changes in the alkali-treated membranes were measured and the effects on membrane performance were investigated thoroughly by analyzing FTIR spectra, water-splitting capabilities, and current-voltage curves. The FTIR spectra showed that the quaternary ammonium groups in the anion-exchange layer (AEL) of the BP-1 membrane decomposed via the Hofmann reaction when the membranes were immersed in 5.0 M NaOH at over 40 degrees C, and in 2.5 M NaOH at 50 degrees C. In addition, the water-splitting capability of the membranes that had undergone structural change decreased. This was attributed to decreased Donnan exclusion resulting from the decomposition of functional groups in the AEL of the BP-1 membrane. The results of the current-voltage experiments were similar to those of FTIR and water-splitting capability studies. The electric resistance for water dissociation increased when the structure of the AEL changed in concentrated alkaline solution, and at high temperature. We concluded that a more stable bipolar membrane anion-exchange layer is required for use in concentrated alkaline solutions. (C) 2005 Elsevier B.V. All rights reserved.