The crystallization of amorphous iron(III) hydroxide during hydrothermal treatment in aqueous suspensions was studied by x-ray diffraction and transmission electron microscopy. The results demonstrate that, by varying the hydrothermal synthesis conditions (pH, temperature, duration, nature and amount of additives), one can control the phase composition, shape, and size of the forming particles. Factors that increase the concentration of soluble iron(III) hydroxocomplexes (e.g., an increase in pH) favor the formation of α-FeOOH particles, and vice versa, a reduction in the concentration of such complexes (e.g., by introducing complexing agents) leads to the formation of α-Fe2O3 particles.