A new rational method of synthesis of nanocrystalline iron (II, III) oxide is based on the elevated-temperature hydrolysis of chelate iron alkoxide complexes in solutions of corresponding alcohol, diethylene glycol, and N-methyl diethanolamine. The rates of reaction and crystallization are easily tuned by varying the concentration of reactants and changing the temperature. The size of the nanocrystals is controlled by changing the complexing strength of the reaction medium: 5.7 nm in diethylene glycol, 16.8 nm in N-methyl diethanolamine, and 12.7 nm in a 1:1 mixture of both solvents. The nanocrystalline powdered Fe3O4 products were isolated with a high yield. The surface of the obtained nanocrystals is passivated by molecules of adsorbed donating solvent that provide stability against agglomeration, provide solubility in polar protic solvents (water and methanol), and allow reactions at the nanocrystal surface.