The variation of the secondary electron yield (SEY) of sputter-cleaned oxygen-free high-conductivity copper has been studied as a function of air exposure duration at room temperature. After short air exposures of some seconds the maximum SEY (delta(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, delta(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 min air exposure of copper at 350 degrees C followed by a 350 degrees C bakeout under vacuum reduces delta(max) to about 1.05, which is lower than the value of pure copper and that of Cu2O.