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Hydrolysis of the new metallorganic precursor Fe(OBut)2(THF)2 followed by ultrasound and thermal treatment yielded either nanoparticles of γ-Fe2O3 or Fe3O4, depending on the process conditions. The samples have been characterised by X-ray powder diffraction, TEM, SEM, Mossbauer spectroscopy and magnetization measurements. The maghemite nanoparticles (9 ± 2 nm) are superparamagnetic and form unique needle-like assemblies of nanoparticle arrays. The nanoparticles of Fe3O4 (19 ± 2 nm) form plate-like aggregates about 10 µm thick.