We produced magnesium ferrite (MgFe2O4) nanoparticles by hydrothermal synthesis in supercritical water. Suspensions containing varying ratios of Mg(OH)(2) and Fe(OH)(3) at room temperature were pressurized to 30 MPa, fed into a tubular reactor by high-pressure pump, and rapidly heated to reaction temperature by mixing with supercritical water. The MgFe2O4 phase forms at 460 degrees C. The Mg/Fe molar ratio was varied from 0.5 to 1.5 with the goal of obtaining single-phase MgFe2O4. At the stoichiometric ratio for MgFe2O4, Mg/Fe = 0.5, the product contains both MgFe2O4 and alpha-Fe2O3. At Mg/Fe = 1.0 and 1.5, the product is the desired single-phase MgFe2O4. The synthesized MgFe2O4 nanoparticles, with particle size of about 20 nm, exhibit superparamagnetic behavior. (C) 2009 Elsevier B.V. All rights reserved.