We present experimental evidence of a previously unknown aluminum oxidation state due to chemisorbed atomic oxygen. This new oxidation state has been detected at room temperature on the chemically shifted Al 2p core level by photoemission spectroscopy. The state was already obtained at submonolayer coverage by a nonconventional deposition of aluminum oxide on cleaved Si. At low metal coverage, only this partially oxidized state is visible, whereas at higher deposition the clean Al 2p doublet is accompanied by the standard alumina peak and other two substoichiometric oxide configurations, shifted to larger binding energy. The Al 2p spectra indicate the growing of clusters or islands with a large fraction of Al-O bonds. These results have been confirmed by similar experiments performed on other substrates (SiO2, graphite), showing that this Al intermediate oxidation state is substrate independent but is confined at the interface.