In situ reduction of nickel oxide (NiO) particles is performed under 1.3 mbar of hydrogen gas (H2) in an environmental transmission electron microscope (ETEM). Images, diffraction patterns and electron energy-loss spectra (EELS) are acquired to monitor the structural and chemical evolution of the system during reduction, whilst increasing the temperature. Ni nucleation on NiO is either observed to be epitaxial or to involve the formation of randomly oriented grains. The growth of Ni crystallites and the movement of interfaces result in the formation of pores within the NiO grains to accommodate the volume shrinkage associated with the reduction. Densification is then observed when the sample is nearly fully reduced. The reaction kinetics is obtained using EELS by monitoring changes in the shapes of the Ni L2,3 white lines. The activation energy for NiO reduction is calculated from the EELS data using both a physical model-fitting technique and a model-independent method. The results of the model-fitting procedure suggest that the reaction is described by Avrami models (whereby the growth and impingement of Ni domains control the reaction), in agreement with the ETEM observations.