Abstract The surface reactions of thin films of Np metal with molecular and atomic oxygen were investigated by X-ray and Ultra-Violet Photoelectron Spectroscopy (XPS and UPS, respectively). Goal of this work was to study the entire range of oxides, starting with the very early reaction stages, in presence of metal, up to the highest possible oxides, reached at saturation under highly reactive, oxidative conditions. Emphasis was given to the surface layers, whose properties often differ from the bulk, and which are directly involved in corrosion processes of solids. Molecular O 2 reacts readily with the metallic neptunium surface to form the sesquioxide and dioxide. The sesquioxide is observed as thin ‘bulk’ species of up to nine monolayers thickness. A higher oxide, identified as Np 2O 5, is formed when the NpO 2 surface is exposed to atomic oxygen. It is stable under UHV conditions up to a temperature of about 200 °C. The high oxide, Np 2O 5, is still capable of chemisorbing further oxygen. This is shown in UPS spectra by the additional O-2p line at 5–6 eV BE, superimposing onto the valence band (VB). The formation of both a lower (Np 2O 3) and higher oxide (Np 2O 5) besides the dioxide is discussed in the framework of ongoing 5f localization throughout the actinide series.