Iron trihydroxide has been prepared by the oxidation of FeSO4 with [Co(III)(en)(dien)]2O2[ClO4]4, [Co(III)(tetraen)]2O2[ClO4]4, and H2O2 in acid solution. These materials have been characterized by DTA, magnetic susceptibility, Mössbauer spectroscopy. X-ray powder diffraction, and electron microscopy. Particle sizes are found to be ~80 Å in diameter, but in the case of the (tetraen) preparation we find needle-like crystallites ~1200 Å × 400 Å × 50 Å in size. The magnitudes of the hyperfine fields follow the particle size variation. These materials have quite a different behaviour from the Fe(OH)3 gels prepared from basic solution.