At room temperature in the absence of gas-phase oxygen, reduced Cu on Cu/ZnO extracts oxygen from the ZnO lattice to reoxidize the surface. After 120 min at room temperature, diffusion of lattice oxygen reoxidizes reduced Cu/ZnO to 3% of its completely oxidized state. When gas-phase oxygen is present, it promotes the partial reoxidation of the reduced Cu/ZnO surface at room-temperature.