Oxidation states of AlOx in Co/AlOx/Co spin tunneling junctions were studied using X-ray photoelectron spectroscopy. Co(10nm)/AlOx/Co(50nm) spin tunneling junctions were fabricated on glass substrates by ion-beam mask sputtering. The Al layers were deposited in various thicknesses and were oxidized in pure O2 gas for 24-48 hours at room temperature. Co/AlOx bilayers simultaneously fabricated were analyzed by XPS. The thicker the Al layer is deposited, the more the unoxidized Al remains. MR ratio of the junctions increasedas the unoxidized Al decreased, and this result is qualitatively in agreement with the LMTO calculation result. The O/Al ratio of AlOx layer is about 1.9-2.0, which means existence of the higher order oxides than Al2O3. XPS depth profiles showed that the unoxidized Al remains not only near the interface with Co but also near the surface when the deposited Al layer thickness isthick.