In a first experiment, we subjected a mixture of Fe3O4 and Fe (80-20 wt.%) to mechanochemical activation by high-energy ball milling, for time periods ranging from 0.5 to 14 h. Complementary X-ray diffraction (XRD) and Mossbauer spectroscopy data demonstrated a phase transformation of magnetite to hematite, accompanied by a partial oxidation of iron to hematite. The reaction can be used to obtain nanometer-size magnetite by ball milling, due to the inhibition of its transformation to hematite, caused by the presence of iron atoms. In a second experiment, we ball-milled Fe3O4:Co2+ (Fe3-xCoxO4 with x = 0.1) for time intervals between 2.5 and 17.5 h. Our XRD and Mossbauer measurements showed that the cobalt-doped magnetite undergoes a phase transformation to hematite, which is actually cobalt-doped hematite. We were able to show herewith that the Co ion is not kicked out of the lattice during the milling process, but undergoes the phase transformation inside the hematite lattice. Finally, we exposed a mixture of Fe3O4 and Co (80-20 wt.%) to mechanochemical activation for time periods ranging from 0.5 to 10 h. The XRD and Mossbauer results are consistent with the formation of cobalt ferrite (a strongly Co-substituted magnetite), with the occurrence of hematite as an intermediate product. In all three cases, the milling-induced phase transformations started with a considerable disorder of the octahedral sublattice of magnetite. (C) 2003 Elsevier B.V. All rights reserved.