Low-temperature oxidation of a copper single crystal, Cu(1 1 1), was investigated using an in situ spectroscopic ellipsometer. The oxidation rate followed the cubic rate law at 5-25 nm oxide thickness; thus, the rate law of Cu single crystal oxidation depended on Cu oxide thickness. Furthermore, the activation energy was found to be close to that of grain boundary diffusion of metal ions in the oxide layer. These results could be explained by grain boundary diffusion and oxide grain growth. Thus, we verified that the low-temperature oxidation kinetics of copper depend on oxide grain growth. (C) 2013 Elsevier B.V. All rights reserved.