Abstract Oxygen absorption in the light exhibited by spinach chloroplasts is traditionally explained by assuming that oxygen functions as Hill acceptor (Mehler, A.H. (1951) Arch. Biochem. Biophys. 34, 339–351). In this paper it is suggested that the light-driven absorption of oxygen results in the production of superoxide and that this process is catalyzed by a 58 kDa catalase. Superoxide then dismutates to hydrogen peroxide and oxygen. It is further suggested that the production of hydrogen peroxide takes place in the lumin of the thylakoid and that the hydrogen peroxide is used as electron donor for photosynthesis. There will only be little loss of oxidizing equivalents, since the oxygen-evolving complex does not have to reach higher oxidation steps. This will be an advantage for light-limited plants. The use of hydrogen peroxide as electron donor will have the consequence that the production of oxygen is higher on the second flash in a series of flashes than when water is used as electron donor. It is also suggested that hydrogen peroxide can deactivate the higher oxidation steps in the oxygen-evolving complex.