The particle mean size of magnetite precipitated in aqueous solution can be adjusted and stabilized against ripening over a large range at the nanometric scale (1.5-12.5 nm). Such a tailoring of particles is obtained by controlling the pH and the ionic strength imposed by a noncomplexing salt in the precipitation medium. The higher the pH and the ionic strength are, the smaller the particle size is. Above a critical pH value, which depends on the ionic strength and the temperature, the secondary particle growth by Ostwald ripening does not take place anymore. The stabilization of nanoparticles seems to result from thermodynamics rather than kinetics.