Al2O3 thin films were grown on highly-doped p-Si (100) macro- and mesoporous structures by atomic layer deposition (ALD) using trimethylaluminum (TMA) and water H2O as precursors at 300 °C. The porous silicon (PSi) samples were fabricated utilizing a metal-assisted chemical etching process (MACE). The morphology of the deposited films and initial silicon nanostructures were investigated by means of scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDX). X-ray photoelectron spectroscopy (XPS) was used to analyze the chemical elemental composition by observing the behavior of the Al 2p, O 1s and C 1s lines. Calculated Auger parameter and binding energy analysis confirmed Al2O3 formation. The measurement of band gap energies of Al2O3 was performed.