The structure of alumina (Al ₂O ₃) films of various thicknesses grown by the atomic layer deposition method on porous silica (por-SiO ₂) substrates has been studied using soft X-ray reflection spectroscopy. It is established that the synthesized films are amorphous and that the ratio of Al atoms with tetrahedral and octahedral coordinations in a film depends on its thickness. It can be suggested that thicker Al ₂O ₃ films contain a greater proportion of Al atoms with tetrahedral coordination.