The effect of electron conditioning on commercially available aluminium alloys 1100 and 6063 was investigated. Contrary to the assumption that electron conditioning, if performed long enough, can reduce and stabilize the secondary electron yield (SEY) to low values (<= 1.3, the value for many pure elements), the SEY of aluminium did not go lower than 1.8. In fact, it reincreased with continued electron exposure dose. The SEY was monitored as a function of electron dose and the surface chemistry was measured with x-ray photoelectron spectroscopy (XPS). The XPS carbon and aluminium core levels showed that the late increase in SEY is due to electron desorption of adsorbed gas, thereby exposing high-SEY Al2O3.(c) (c) 2005 American Vacuum Society.