The kinetics of the reactions between H 2O 2 and I − were studied in artificial seawater as a function of temperature (10–30 °C), salinity (0–36), pH (7–9) and the concentrations of molybdate (0–480 μM) at a range of concentrations of H 2O 2 (5 to 40 mM) and I − (0.1 to 30 mM) by following the changes in the concentrations of the two reactants with time. While the rate of disappearance of H 2O 2 was accelerated significantly in the presence of I −, the concentration of I − remained constant in all the experiments even when the amount of H 2O 2 lost had far exceeded the amount of I − present. This catalytic decomposition of H 2O 2 by I − was consistent with the reaction scheme in which H 2O 2 first oxidized I − to I 2 and then reduced the I 2 formed back to I −. The disappearance of H 2O 2 was first order with respect to both the concentration of I − and H 2O 2 so that the rate law could be expressed as − d[H 2O 2] / d t = k[I −] [H 2O 2] where [I −] and [H 2O 2] were the concentrations of I − and H 2O 2 in mM and k, which was equal to 0.0600 mM − 1 h − 1 at a pH of 8.0, a temperature of 20 °C and a salinity of 35.7 in artificial seawater, was the second order reaction rate constant. Within the oceanographic range, k was essentially independent of pH. At a constant concentration of I − of 0.5 mM, the rate constant was linearly related to salinity S so that: k I = 0.00012 ( ± 0.00001 ) S + 0.0283 ( ± 0.0003 ) wherer k I = k[I −]. Its dependence on temperature, T in K, follows the form of the Arrhenius relationship so that: ln k I = − 6865 ( ± 84 ) ( 1 / T ) + 19.96 ( ± 0.02 ) When these results were extrapolated to the conditions found in surface seawater, the estimated half life of H 2O 2 was much longer than the observed values, suggesting that the reactions between H 2O 2 and I − are not an important determining factor of the fate of H 2O 2 in seawater. The impact of these reactions on the marine geochemistry of iodine will depend on the fate of the I 2 formed in the forward reaction. Molecular iodine is a reactive transient and it may participate in other reaction pathways in seawater where forms of iodine other than I − can be formed. Under the experimental conditions used, there was no evidence that the presence of molybdate at the concentrations found in seawater increased the rate of the reactions between H 2O 2 and I −.