To further enhance the catalysis of hyperbranched aliphatic polyester Boltorn (R) H30 on water dissociation, it was carboxylated before introduced to the intermediate layer of a bipolar membrane. At a current density of 100 mA/cm(2), the membrane modified with Boltorn (R) H30-COOK can have a voltage drop 1.8 V lower than the membrane modified with Boltorn (R) H30. As for the end groups for modification, -COOK is a better choice than -COOH and -OH because it is the most hydrophilic and the most capable to form intermolecular hydrogen bonds with water molecules. (C) 2009 Elsevier B.V. All rights reserved.