X-ray photoelectron spectroscopy (XPS) results referring to the interface formed upon vapor deposition of Ni atoms on NiO(001) indicate, that upon annealing the deposited Ni is oxidized forming a NiO overlayer. The oxidation process is activated and becomes very fast above 900 K, resulting in a surface that exhibits identical photoelectron spectrum with the clean substrate. In addition, it was found that during the oxidation process two extra components in the O 1s XPS peak spectra appear, with binding energies above and below the main lattice oxygen peak, manifesting the existence of oxygen atoms carrying charges that deviate from their formal ionic value. Moreover, by molecular dynamics simulations it was found that upon deposition of Ni ions, neighboring O surface atoms become adatoms and combine with the Ni ions forming a series of NixOy oxides, most of which eventually coalesce into small NiO islands. Consequently, the lower binding energy component of the 0 1s XPS peak may be attributed to intermediate oxides with x > y, while the higher binding energy component to intermediate oxides with x < y. (C) 2003 Elsevier Science B.V. All rights reserved.