Extraordinary Hall effect and x-ray spectroscopy measurements have been performed on a series of Pt / Co /M Ox trilayers ( M= Al , Mg, Ta, etc.) in order to investigate the role of oxidation in the onset of perpendicular magnetic anisotropy at the Co /M Ox interface. It is observed that varying the plasma oxidation time modifies the magnetic properties of the Co layer, inducing a magnetic anisotropy crossover from in plane to out of plane. We focused on the influence of plasma oxidation on Pt/Co/AlOx perpendicular magnetic anisotropy. The interfacial electronic structure is analyzed via x-ray photoelectron spectroscopy measurements. It is shown that the maximum of out-of-plane magnetic anisotropy corresponds to the appearance of a significant density of Co–O bondings at the Co/AlOx interface.