The initial yields of H2O2 in aerated water, [Formula: see text] and in water containing H2 and O2, [Formula: see text] have been measured for alkaline solutions irradiated with 60Co γ-rays. [Formula: see text] decreases with increasing pH from a value of 1.22 in neutral solution to 0.63 in solutions of pH 13.92 and the relationship[Formula: see text]is valid over the pH range 7 to 14.[Formula: see text] decreases from 3.30 in neutral solution to a minimum of 2.00 at pH 11.35 and then increases to 2.65 at pH 13.92. The equation[Formula: see text]which is applicable for neutral solutions, is not valid for basic solutions.A reaction mechanism in accordance with the observed results and the literature values of the rate constants of likely radical reactions has been developed. It is assumed that in the alkaline solutions investigated OH radicals react with OH− ions to form O− radicals which react preferentially with O2 to form O3− radicals which then react either with H2 or H2O2. The increase in [Formula: see text] at PH > 12 is attributed to a difference in the rate of reaction of O3− with H2O2 and HO2−, k(O3− + H2O2)/k(O3− + HO2−) = 2.45.