The paper discloses the mechanism of the reactions of oxygen reduction at the negative plate involved in the oxygen cycle in VRLA batteries. Experimental methods have been developed for determining both the current of the electrochemical reduction of oxygen and the current of the oxygen cycle. Through these methods it has been established experimentally that the reduction of oxygen that results in the formation of water proceeds through an electrochemical mechanism of oxygen reactions and a chemical mechanism of reactions between the intermediate products of the oxygen and hydrogen reactions at the negative plate. A general mechanism has been proposed which involves the elementary reactions of oxygen reduction and of hydrogen evolution. The first stage of the reduction of oxygen results in formation of hydrogen peroxide that reacts (through a chemical reaction) with the atomic hydrogen evolved by the decomposition of water (chemical mechanism of water formation). Hydrogen peroxide can be reduced to H2O by an electrochemical reaction as well (electrochemical mechanism of water formation). Both mechanisms of water formation (chemical and electrochemical) are in competition as they use the same intermediate product (H2O2) of the oxygen reaction. At low temperatures, the electrochemical mechanism is the dominating one and at high temperatures, it is the chemical mechanism. A modified version of the general mechanism of oxygen reduction is proposed in which OH radicals and oxygen atoms are formed as intermediate products. Which of the above mechanisms will be activated, depends on the potential of the negative plate, the temperature and the catalytic properties of the lead surface (and of the alloying additives used) as well as on the saturation of the negative plate with electrolyte. The proposed mechanism indicates that the hydrogen reaction plays an important role in determining the efficiency of the oxygen cycle. (c) 2004 Elsevier B.V. All rights reserved.