The changes in magnesium ferrite. MgFe2O4, due to high-energy milling in a stainless steel vial have been investigated by Mossbauer spectroscopy, X-ray diffraction, electron microscopy, and thermal analysis. The milling process reduces the average crystallite size of MgFe2O4 to the nanometer range. Prolonged mechanical treatment leads to the chemical reduction of MgFe2O4 and to the formation of a solid solution of FeO and MgO. In addition to the solid Solution. metallic iron has been found as a byproduct. The fraction of the reaction products increases with increasing milling time. The range of thermal stability of the metastable milled reduction products has been determined by studying their response to changes in temperature.