The Al redistribution in the SiO2/Si system was investigated during thermal oxidation of high dose Al implanted Si wafers. Only a small fraction of the implanted Al atoms remains electrically active in the substrate, and this amount decreases when thicker oxides are grown. About 50% of the implanted Al is distributed inside the growing oxide at a concentration of about 10(20) cm(-3) Al atoms are embedded in the SiO2 layers through two distinct mechanisms: the dopant redistribution during oxidation, due to the low Al segregation coefficient, and the inclusion of small Al-containing precipitates formed inside the Si substrate due to the low Al solubility and its great reactivity with oxygen. Furthermore, a relevant Al fraction evaporates during the first stages, including the temperature ramp-up, of the oxidation process. Also, the inclusion of Al-containing precipitates in the SiO2 layer leads to the formation of very rough oxide surfaces.