Dispite a few studies on the Cu─O eutectic bonding of copper and alumina, the mechanism of Cu─Al2O3bonding has been unclear to the present, especially with respect to the formation of a reaction phase. The present work reveals that peel strngth decreases and recovers with the concentration of oxygen at the interface, and that the recovery process is rate‐controlled by the diffusion of oxygen in copper. There is a thin, 5‐nm‐thick, reaction layer between the copper and the alumina, which is likely to be amorpous. Incorporating XPS data of the interface, thermodynamic considerations suggest that the reaction phase is CuAlO2. The variation of peel strength is discussed and related to the stability of CuAlO2and oxygen at the