Synthesis of undoped and transition metal ion doped Mn3O4 microcrystals were achieved through a simple hydrothermal route. The morphologies and structures of the obtained products were characterized using X-ray diffraction, X-ray photoemission spectroscopy and scanning electron microscopy. Results revealed that the low volume percentage of ethanol in the precursor solution limited formation of Mn2+, while the introduction of doping ions into the precursor solution caused a direct synthesis of single phase Mn3O4 crystals. For Cu and Ni doping ions, the spontaneous valence changes during the doping process were taken place. The possible doping mechanisms for the formation of single-phase Mn3O4 were discussed briefly. UV-vis spectroscopic studies showed an apparent change in the energy bandgap of Mn3O4 with the metal doping. (C) 2013 Elsevier Ltd. All rights reserved.