The structure and morphology of thin aluminium-oxide films grown by the dry, thermal oxidation of a bare Al(4 3 1) substrate at a partial oxygen pressure of 1.33 X 10(-4) Pa in the temperature range of 373-773 K were studied using X-ray photoelectron spectroscopy and high resolution electron microscopy. The initial oxidation of the bare Al substrate proceeds by an island-by-layer growth mechanism, involving the lateral diffusion over the bare Al substrate surface of mobile oxygen species. At low temperatures (Tless than or equal to573 K), the mobility of the oxygen species is very low, and an amorphous oxide film of relatively uniform, limiting thickness develops. X-ray photoelectron spectroscopic analysis established the occurrence of a surface-oxide species at the very surface of these films. At higher temperatures (T>573 K) an initially amorphous oxide film of less uniform thickness develops that gradually transforms into crystalline gamma-Al2O3. At these temperatures an amorphous-to-gamma-Al2O3 transition oxide phase occurs.