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Hausmannite (Mn₃O₄) nanoparticles have been prepared by mixing aqueous solutions of manganese nitrate and hexamethylenetetramine from 20 to 80 °C. Activation energy for the particle formation increases from 0.5 to 0.8 kJ/mol with nitrate concentration. Nanoparticles (18−41 nm) with a faceted structure are prepared by this method. We describe synchrotron in-situ time-resolved XRD experiments in which Mn₃O₄ nanoparticles are reduced to MnO and subsequently reoxidized in ramping temperature conditions. The temperature of Mn₃O₄ to MnO reduction decreases as Mn₃O₄ particle size decreases. On oxidation, 18 nm and smaller MnO nanoparticles formed the intermediate phase Mn₅O₈ (MnO → Mn₃O₄ → Mn₅O₈ → Mn₂O₃), while larger MnO particles oxidized to Mn₃O₄ then directly to Mn₂O₃. Formation of Mn₃O₄ occurred at lower temperature for smaller MnO nanoparticles. Further oxidation to Mn₂O …