Redox properties of Keggin-type heteropolyacid (HPA) catalysts were determined with the aim of providing a reduction potential database of these compounds. Cation-exchanged, polyatom-substituted, and heteroatom-substituted HPAs were examined to investigate the effects of different substitutions. Reduction potentials of HPA samples were determined by electrochemical methods. The reduction potentials of HPA catalysts could be correlated with the electronegativity of the substituted atoms. Substitution of more electronegative atoms for counter-cations or for the central heteroatom increased reduction potentials of the HPAs. However, substitution of more electronegative metals into the Keggin framework decreased reduction potentials. This work demonstrates how one can estimate or predict reduction potentials of Keggin-type HPA catalysts. A map of reduction potentials of HPA catalysts was established to provide a design basis in searching for catalytic oxidation processes using HPAs. (C) 2003 Elsevier B.V. All rights reserved.