Oxidation potentials of solid copper crystals have been evaluated for the isolated crystallographic planes (111), (110), (100), (210). It has been demonstrated that the only stable surface configuration of atoms is that of the close‐packed octahedral planeand, accordingly, only this plane on the electrode surface produces a reversible cell. The significance of potentials obtained with other forms of electrodes has been interpreted on the basis of the observed crystallographic requirement for equilibrium.