Magnetite nanoparticles were synthesized by electrocrystallization in the presence of thiourea or sodium butanoate as an organic stabilizer. The synthesis was performed in a thermostatic electrochemical cell containing two iron electrodes with an aqueous solution of sodium sulfate as electrolyte. The effects of organic concentration, applied potential and growth temperature on particle size, morphology, structure and magnetic properties were investigated. The magnetite nanoparticles were characterized by X-ray diffraction, electron microscopy, magnetometry and Mossbauer spectrometry. When the synthesis is performed in the presence of sodium butanoate at 60 degrees C, a paramagnetic ferric salt is obtained as a second phase; it is possible to avoid formation of this phase, increase the specific magnetization and improve the structure of the oxide particles by tuning the growth conditions. Room-temperature magnetization values range from 45 to 90 Am(2)kg(-1), depending on the particle size, type of surfactant and synthesis conditions. Mossbauer spectra, which were recorded at 290 K for all the samples, are typical of nonstoichiometric Fe3-delta O4, with a small excess of Fe3+, 0.05 <= delta <= 0.15.