Experiments are described which show that at all current densities the electromotive force of alkaline air‐depolarizing electrodes is that thermodynamically required by equilibrium between oxygen and peroxide ion. In acidic solutions, however, the reduction of oxygen to hydrogen peroxide at carbon electrodes is irreversible. A tentative explanation for this is offered. In acidic and in alkaline air‐depolarized cells hydrogen peroxide disappears by catalytic decomposition alone. The mechanism by which oxygen passes through air‐depolarizing electrodes is shown to be diffusion, and a method for obtaining the diffusion coefficient is described.