The variation of the secondary electron yield (SEY) of sputter-cleaned OFHC-copper has been studied as a function of air exposure duration at room temperature. After short air exposures of some seconds the maximum SEY (δ MAX ) of clean copper is reduced from 1.3 to less than 1.2, due to the oxidation of the copper surface. Prolonged air exposure increases the SEY steadily until, after about 8 days of atmospheric exposure, δ MAX is higher than 2.Air exposures at higher temperatures have been found to be effective in reducing the SEY of technical copper surfaces. A 5-minute air exposure of copper at 350°C followed by a 350°C bake-out under vacuum reduces δ MAX to about 1.05, which is lower than the value of pure copper and that of Cu 2 O.