A study was made of electrochemical behavior of aluminum in the presence of aqueous solutions. The potential pH equilibrium diagram of the system aluminum-water at 25 C was developed from the standard free energies of the constituents, and the general electrochemical behavior of aluminum was deduced from the diagram. The diagram was established by considering the ions Al⁺⁺⁺ and ALO₂ and the solid phases Al and Al₂O₃ * 3 H₂O (hydrargillite). The diagram indicates the theoretical circumstances in which aluminum should show corrosion, immunity, and passivity, under the hypothesis that the passivation results from the formation of a film oxide whose stability resembles that of hydrargillite. The stability of aluminum and the different forms of its oxides are discussed. Curves are given of the solubilities of the oxides and hydroxides as functions of pH.