Ni-doped alumina spinels are known catalysts for hydrocarbons reforming that are typically synthetized by reacting Ni nitrates with gamma-Al2O3. It is believed that Ni particles (the active phase) are reduced from NiA1204 once subjected to reaction. Here, we revisit the mechanisms of Ni particles formation from Ni-alumina catalysts by X-ray diffraction (XRD) and transmission electron microscopy (TEM) of fresh and used catalysts. We also monitored the evolution of the catalyst's crystalline structure exposed to reactants, by in situ XRD. Ni-alumina catalyst structure can be explained by a tetragonal (I4(1)/amd space group) spinel phase with Al (or Ni cations) occupying 4a, 8c and 8d Wyckoff sites. Ni is initially distributed homogeneously in the alumina platelets. The alumina from the fresh catalyst is initially cubic, but under reducing conditions typical to reforming (CO2:CH4= 1:1 at 850 degrees C), the c/a ratio decreases and the sites occupancy changes. After 12-days reaction, facetted nanoparticles are formed and share an interface with tetragonal alumina platelets with the following crystalline relationships: Ni-(110)parallel to NixAlyO4(20 (2) over bar) and Ni-(111)parallel to NixAlyO4(20 (2) over bar). This study brings valuable insight into the activation mechanism and microstructural evolution of a yttria-stabilized-zirconia/Ni-alumina catalyst to understand the behaviour of other spinel catalysts. (C) 2017 Elsevier B.V. All rights reserved.