The electrochemical reduction and oxidation kinetics of hydrogen peroxide on gamma-FeOOH films chemically deposited on indium tin oxide substrates were studied over the pH range of 9.2-12.6 and the H2O2 concentration range of 10(-4) to 10(-2) mol dm(-3). The Tafel slopes for H2O2 reduction and oxidation obtained from polarization measurements are 106 +/- 4 and 93 +/- 15 mV dec(-1), respectively, independent of pH and the concentration of H2O2. Both the reduction and oxidation of H2O2 on gamma-FeOOH have a first-order dependence on the concentration of molecular H2O2. However, for the pH dependence, the reduction has an inverse first-order dependence, whereas the oxidation has a first-order dependence, on the concentration of OH-. For both cases the electroactive species is the molecular H2O2, not its base form, HO2-. Based on these observations, reaction kinetic mechanisms are proposed which involve adsorbed radical intermediates: (HOOH-)-O-center dot and HO center dot for the reduction, and (HO2H+)-H-center dot, HO2 center dot, and O-center dot(2)- for the oxidation. These intermediates are assumed to be in linear adsorption equilibria with OH- and H in the bulk aqueous phase, respectively, giving the observed pH dependences. The rate-determining step is the reduction or oxidation of the adsorbed H2O2 to the corresponding intermediates, a reaction step which involves the use of Fe-III/Fe-II sites in the gamma-FeOOH surface as an electron donor-acceptor relay. The rate constant for the H2O2 decomposition on gamma-FeOOH determined from the oxidation and reduction of Tafel lines is very low, indicating that the gamma-FeOOH surface is a very poor catalyst for H2O2 decomposition. (C) 2010 Elsevier Ltd. All rights reserved.