A reaction-plane model is used to examine the behavior of copper in aerated acid sulfate solution at or near the open-circuit potential. The model predicts that when dissolved oxygen is present, the metal will corrode at a mixed potential determined by the Cu2+/Cu1+ and Cu1+/Cu couples. The mixed potential in an aerated solution is negative to the equilibrium potential in a deaerated solution; it is controlled by the kinetics of the Cu2+/Cu1+ couple and by diffusion of oxygen to a reaction plane where cuprous ion is consumed. The computed potentials are compared with measured open-circuit potentials of copper single crystals with orientations of (100), (110), and (111). Galvanic cells that could arise under this mechanism are explored with the model.