[en] 30 keV O₂⁺ ions are implanted in aluminium foil, at various doses ranging from 5 x 10¹⁶ to 5 x 10¹⁷ atoms cm^-2. The sheet resistance was measured for unimplanted and implanted specimens. Up to the dose of 3 x 10¹⁷ atoms cm^-2, the sheet resistance gradually increased. It formed a perfect insulating layer on the surface, above a dose of 3 x 10¹⁷ atoms cm^-2. X-ray photo-electron spectroscopy measurements indicated the formation of an Al₂O₃ layer with the absence of a signal due to unbonded aluminium for a dose of 5 x 10¹⁷ atoms cm^-2. Whereas a specimen implanted at 5 x 10¹⁶ atoms cm^-2 shows the presence of pure aluminium. Micrographs of the specimen implanted at the higher dose displayed the microtopography characteristic of alumina. The results are in agreement with a recently published mechanism of Al₂O₃ formation. The Al₂O₃ precipitates increasing with dose and a layer of Al₂O₃ being formed at the doses higher than that required to form the stoichiometric Al₂O₃. (author)