Magnetic monodisperse Fe3O4 nanoparticles (NPs) were prepared from ferrihydrite and iron powder by a novel solvethermal method at moderate temperatures in aqueous and organic solvent media. The effects of temperature and solvent medium on the phase and particle size of Fe3O4 NPs were investigated. The products were characterized by X-ray powder diffraction, infrared spectroscopy, differential scanning calorimetry thermogravimetric analysis, transmission electron microscopy, and vibrating sample magnetometry. At 120 degrees C in the organic medium, uniform organic-ligand-coated superparamagnetic Fe3O4 NPs were produced; the particle size increased with the temperature, and the calculated mean particle size was 8.9 nm. At 180 degrees C in the aqueous medium, pure-phase Fe3O4 was obtained. Thus, the organic solvent did not only serve as a solvent but also participated in the reaction. Fe3O4 production in the aqueous and organic solvent media had different mechanisms. (C) 2013 Elsevier Ltd. All rights reserved.