Fe3O4 was pptd. from an aq. soln. of Fe2+ and Fe3+ by addn. of Na2CO3 in presence of poly(ethylene glycol) used to protect the product against oxidn. and agglomeration of nanoparticles. Reaction was carried out at 80-90 degrees C and varying rates of stirring (400-6000 rpm). The presence of Fe3O4 phase was evidenced by X-ray powder diffraction. The increase of reaction temp. and stirring rate resulted in an increase of the ay. crystallite size (from 12 nm to 21 nm).