X-ray spectroscopy measurements have been performed on a series of Pt∕Co∕AlOx trilayers to investigate the role of Co oxidation in the perpendicular magnetic anisotropy at the Co∕AlOx interface. It is observed that varying the degree of oxidation modifies the magnetic properties of the Co layer, inducing a magnetic anisotropy crossover from in plane to out of plane. The microscopic structural properties are analyzed via x-ray photoelectron spectroscopy measurements. It is shown that increasing the oxidation time enhances the amount of interfacial oxide, which may be at the origin of perpendicular magnetic anisotropy.