Using Ellman’s reagent [5, 5′-dithiobis (2-nitrobenzoic acid)], it was shown that thermally activated reducers in seawater are bicarbonate and chloride anions and that their joint effect is superadditive. Sulfate anions do not exhibit such properties. By studying enhanced chemiluminescence in the luminol–p-iodophenol–peroxidase system, the formation of hydrogen peroxide and the effect of seawater anions on the thermally activated hydrogen peroxide production were investigated. In NaCl and NaHCO3 solutions whose concentrations and pH are close to those for seawater, heating increases the H2O2 production more significantly than it increases the H2O2 production in water, with sulfate anions suppressing the formation of H2O2. Using coumarin-3-carboxylic acid, a fluorescent probe for hydroxyl radicals, it was shown that heating considerably increases the OH