As an application of the gel-sol method especially developed for the synthesis of general monodisperse particles in large quantities, uniform hematite (alpha-Fe2O3), magnetite (Fe3O4), and maghemite (gamma-Fe2O3) particles, precisely controlled in size, aspect ratio, and internal structure, have been prepared. For the synthesis of uniform ellipsoidal single-crystal particles of alpha-Fe2O3, a highly condensed suspension of fine beta-FeOOH particles doped with a prescribed amount of pO(4)(3-) ion in their interiors was aged at 140 degreesC for 24 It with seed particles 4 of alpha-Fe2O3 in an acidic medium containing optimum concentrations of HCl and NaNO3. Systematic control of the aspect ratio and mean size was achieved by regulating the concentration of pO(4)(3-) ion incorporated into the beta-FeOOH particles and the number of seeds added. The resulting hematite particles were converted into magnetite by reduction in a H-2 stream at 330 degreesC for 6 h; the magnetite was then oxidized to maghemite in an air stream at 240 degreesC for 2 h. Magnetite and maghemite thus prepared retained the original shape of the hematite. On the other hand, polycrystalline hematite particles of different sizes and aspect ratios were also prepared by aging a condensed Fe(OH)3 gel in the presence of different concentrations of SO42- ion and seeds. The polycrystalline hematite particles were similarly converted into magnetite and then maghemite. The magnetic properties of these magnetite and maghemite particles were analyzed as a function of their mean particle volume, aspect ratio, and internal structure. (C) 2003 Elsevier Inc. All rights reserved.