Highly crystalline and water-soluble magnetite nanoparticles having an average diameter of 6.5 nm were synthesized from aqueous FeCl3 + KOH solution by a flow-through hydrothermal method with a microreactor at 673 K and 30 MPa during residence time of 2 x 10(-3) s. L-Ascorbic acid was used as a modifier and a reductant.