Gamma radiolysis experiments on solutions of a mixture of sodium chloride (NaCl) and sodium bromide (NaBr) were conducted to confirm the validity of radiolysis calculations for simulated seawater solutions and to determine the importance of bromide anion (Br−) in the production of hydrogen peroxide (H2O2) via water radiolysis. The H2O2 concentration in each solution was measured after irradiation and compared with that obtained from radiolysis calculations. It was found that the calculated and experimental results were in good agreement. The concentration of H2O2 in a 0.6 M NaCl solution increased approximately three times on the addition of 1 mM NaBr. The result showed that Br− plays an important role in the production of H2O2 by water radiolysis, presumably through the reactions of Br− with hydroxyl radical (●OH). For 1 mM NaCl solutions, there is a minimum production rate of H2O2 at pH 8, which increases when the pH changes to either lower or higher values. It was considered that the hydrated electron also plays an important role in H2O2 production under these acidic and alkaline conditions.