We studied the Fenton reaction (Fe(II) + H2O2 -> OH + OH- + Fe(III)) in coastal seawater samples by measuring Fe(II) and OH radical concentrations during photochemical experiments. Under coastal seawater conditions, the Fenton reaction was found to be an important pathway for H2O2 decomposition in comparison to direct photolysis, but only a minor source of OH. Experimentally observed H2O2 destruction rate constants (k'H2O2) could be accounted for by reactions of H2O2 with Fe2+ and FeOH+.