Abstract <p>The process of magnesia spinel synthesis based on oxides, hydroxides and nitrates of magnesium and aluminum was investigated. As aluminum hydroxide, a reagent in the form of gibbsite and synthesized bayerite was used. The reactivity of precursors was compared using the effective rate constants calculated according to the Ginstling–Brownstein equation. The possibility of using this equation was confirmed by the straightness of dependencies in its coordinates with a high coefficient of linear approximation. Oxide components interacted at a significantly lower rate compared to hydroxides and salts. The effect of preliminary mechanical activation by the impact-attrition and attrition methods in a planetary (PM) and ball-ring mill, respectively, as well as microwave treatment (2.45 GHz) was analyzed. It was found that the most effective combined method consisted of mechanical treatment of Mg and Al nitrates mixture in PM and subsequent burning in a thermal furnace (1200°C), which led to the formation of an almost monophasic product. Microwave processing occupied an intermediate position in terms of effectiveness. The greatest positive effect of PM treatment associated with the activation of Mg and Al compounds revealed itself in the region of relatively low temperatures (1100–1200°C). For example, at a temperature of 1200°C, the magnesia spinel yield in joint impact treatment of oxides was ~2.5 times higher than in simple mixing. With a further increase in temperature, the effect of pretreatment decreased, since under these conditions the diffusion coefficients increased, which ensured a relatively fast reaction even without pretreatment.</p>