Abstract: The dissolution of synthetic magnetite, maghemite, hematite, goethite, lepidocrocite, and akaganeite was faster in HCl than in HClO4. In the presence of H+, the Cl− ion increased the dissolution rate, but the ClO4− ion had no effect, suggesting that the formation of Fe-Cl surface complexes assists dissolution. The effect of temperature on the initial dissolution rate can be described by the Arrhenius equation, with dissolution rates in the order: lepidocrocite > magnetite > akaganeite > maghemite > hematite > goethite. Activation energies and frequency factors for these minerals are 20.0, 19.0, 16.0, 20.3, 20.9, 22.5 kcal/mole and 5.8 × 1011, 1.8 × 1010, 7.4 × 107, 5.1 × 1010, 2.1 × 1010, 3.0 × 1011 g Fe dissolved/m2/hr, respectively. The complete dissolution of magnetite, maghemite, hematite, and goethite is well described by the cube-root law, whereas that of lepidocrocite is not.