A quantitative study has been made of the formation of hydrogen peroxide when alkaline solutions of tannic acid, gallic acid and a number of other molecules containing the catechol (1,2-dihydroxybenzene) moiety are exposed to air or dioxygen. For gallic acid, from the pH dependence of the initial rates of reaction, it is shown that, over the pH range ca. 7.0–8.5, the predominant reactive species is the dianion. The photochemical production of hydrogen peroxide, using 254 nm radiation, has been studied for a variety of air-saturated solutions of phenolic substances at pH 6.0 or below, where the thermal reaction is negligible, and apparent quantum yields are reported. Appreciable formation of hydrogen peroxide is observed with 1,2-, 1,3- and 1,4-benzene diols. Possible mechanisms for the thermal and photochemical reactions are discussed and it is suggested that benzenediol groups may be involved in the natural photochemical production of hydrogen peroxide in surface waters.