The quantification of surface and subsurface oxidation of Be/Al periodic multilayer mirrors due to exposure in the ambient atmosphere was investigated by depth-resolved X-ray photoelectron spectroscopy. The contribution of oxidation was lower for the thicker layer of Al in the periodic structures since the surface was less chemically reactive for the oxidation. This was investigated by finding the depth-resolved slope of the intensity ratio of metal/oxides (Be/BeOx and Al/AlOx) by analyzing the chemical shift of Al 1s and Be 1s photoelectrons. Furthermore, a well-resolved doublet chemical shift in the O 1s spectra indicated the formation of BeOx/AlOx and BeOH/AlOH oxides. The investigation showed that the subsurface and surface regions were dominated by metal-hydroxide (BeOH/AlOH) and metal-oxide (BeOx/AlOx) bonding, respectively, analyzed by the depth-resolved chemical shifts.