X‐ray reflection spectroscopy and x‐ray emission spectroscopy using electron excitation were used to characterize the 30 nm‐thick Al2O3 film grown on a silicon substrate by atomic layer deposition (ALD). According to our investigations the film was amorphous and inhomogeneous in depth. The interfacial layer between film and substrate that was a mixed layer containing Al2O3, a large amount of the SiO2 and elemental Si was evidenced. The concentration of SiO2 increases as one approaches the Si(100) substrate interface because of the interdiffusion of the oxygen and silicon, followed by silicon oxidation. Copyright © 2006 John Wiley & Sons, Ltd.