The sonolysis of an aqueous Solution of Fe(CO)(5) in the presence of sodium dodecyl sulfate leads to the formation of a stable hydrosol of amorphous Fe3O4 nanoparticles. The amorphicity of iron oxide nanoparticles was determined by X-ray diffraction and differential scanning calorimetry. The nanoparticles were characterized by elemental analysis, EDX, transmission electron microscopy, dynamic light scattering, Raman spectroscopy, XPS, and spot test. (c) 2004 Elsevier Inc. All rights reserved.