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The dynamics of the reduction reaction of NixMg1−xAl2O4 to form nickel metal and a remnant oxide was quantified to understand spinel behavior in catalysis applications. X-ray diffraction, thermogravimetry, and pycnometry were employed to track the evolution of high-Ni spinels to metastable nonstiochiometric spinels during reduction, but before the phase transformation to theta alumina. Rietveld refinements of X-ray diffraction data were used to quantify structural changes in the spinel and the phase fraction, crystallite size, and microstrain of all phases during H2 reduction. During reduction, one O2− is lost for each Ni2+ reduced to Ni metal. Ni0.25Mg0.75Al2O4 and Ni0.5Mg0.5Al2O4 were shown to form Ni metal and a non-stoichiometric spinel of the same Mg-Al ratio as the starting composition. NiAl2O4 and Ni0.75Mg0.25Al2O4 were found to become unstable as full reduction was approached, and metastable spinel, Θ-Al2O3, and α-Al2O3 formed sequentially given sufficient time at temperature.