The first stages of nickel oxidation were followed mainly by in-situ oxidations on an heating stick in an XPS apparatus. First the samples, after sputtering of the contaminated surface, were heated upto 200 or 600 degreesC in ultra vacuum ((similar to) 10(-6) Pa), then oxygen was introduced at I Pa (10(-5) atm) fo times varying from 30 s to 15 min. All samples were observed by scanning electron microscopy (SEM) and by optical interferometry allowing to establish 2D-profiles and 3D-images. Some of them were observed by AFM. Thin foils after short exposures at 650 degreesC under low partial pressures were observed by transmission electron microscopy (MET), The results indicate that the oxide is mainly made up of NiO, with a sub-layer and islands growing on the outer surface. The amount of islands is related to the crystallographic orientation of the Ni grains. Extrusion of metallic nickel occurs along the grain boundaries of the substrate.