The oxidation of nickel particles was studied in situ in an environmental transmission electron microscope in 3.2 mbar of O-2 between ambient temperature and 600 degrees C. Several different transmission electron microscopy imaging techniques, electron diffraction and electron energy-loss spectroscopy were used to study the evolution of the microstructure and the local chemical composition of the particles during oxidation. Our results suggest that built-in field effects control the initial stages of oxidation, with randomly oriented NiO crystallites and internal voids then forming as a result of outward diffusion of Ni2+ along NiO grain boundaries, self-diffusion of Ni2+ ions and vacancies, growth of NiO grains and nucleation of voids at Ni/NiO interfaces. We also observed the formation of transverse cracks in a growing NiO film in situ in the electron microscope. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.