Ultrafine alpha-Fe2O3 and Fe3O4 particles were prepared through a hydrothermal process at 150 degrees C from a (NH4)(2)SO4 . FeSO4 . 6H(2)O solution in the presence of hydrazine. These products were characterized by X-ray diffraction and transmission electron microscopy. The formation of pure alpha-Fe2O3 and Fe3O4 powders was strongly influenced by the pH values of the aqueous solution.