The oxidation of a Cu/SiO₂ catalyst with NO or O₂ at temperatures varying from 313 to 513 K has been investigated using XPS. Upon admission of NO or O₂ to a previously reduced catalyst, a rapid transformation of Cu(0) to Cu(I) is observed. This first stage is followed by slow oxidation of Cu(I) to Cu(II). After exposure to NO the presence of a surface nitride at the catalyst surface is evident. Exposure at low temperatures also leads to the formation of nitrito and nitrato like surface species. These species decompose at 523 K. Desorption of the surface nitride is observed after heating the catalyst at 723 K. The XPS measurements performed on the supported catalyst are in good agreement with experimental data obtained on single crystals.