Compared are interfaces between thin silicon oxide layers and two substrates bulk silicon and porous silicon. About 2 mu m thick porous silicon was prepared by an electrochemical etching of Si in HF solution. Oxide layers were formed by thermal oxidation. Part of SiO2/Si samples were covered with 50 nm RuO2 oxide. SIMS depth profiles of these interfaces were performed using ultra-low energy (880 eV) and low energy (5 keV) argon ion beams at several incidence angles. Positive and negative secondary ions were analysed by quadrupole mass spectrometer QMA 410 Balzers during Ar+ bombardment with the use of 06-350E Physical Electronics ion gun. Charge build-up effects during positive secondary ion detection were observed dependent of the thickness of silicon oxide layer. The SiO2/Si interface regions characterised by SIMS show differences related to the kind of a substrate used. In case of 50 nm SiO2/porous Si interface, characteristic oxygen depletion region (20 run thick) is present on the porous silicon side. The observed depletion was confirmed also by Auger depth profiling. The depletion formation mechanism is explained on the basis of ion beam induced redeposition of oxygen inside the pores. (c) 2006 Published by Elsevier B.V.