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The defect microstructure of NiO scales, formed by oxidation of pure Ni substrates at 1275 K in 3 kPa p(O2) for times between 5 s and 25 h, has been studied by transmission electron microscopy (TEM) of sections cut and thinned parallel and transverse to the NiO/Ni interface. It has proven possible in both sectioning techniques to preserve portions of the Ni substrate, so that the NiO/Ni interface itself could be studied. The NiO/oxygen interface was additionally retained in transverse sections by electroplating a Ni layer onto the NiO scale surface before sectioning and thinning by argon ion bombardment. The microstructure of scales invariably comprises a submicron layer of fine-grained NiO topotactically related to the substrate in several orientation relationships and overlaid by thick columnar-grained oxide resulting from subsequent renucleation and competitive growth. Enhanced oxidation is observed at the emergence of NiO grain boundaries at the scale surface. Interface structure is observed ...