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Iron compounds are generously distributed in nature. They play an important role in water supplies, waste waters and in limnology. Water supply personnel are frequently confronted with problems, such as rusty water, corrosion, deferrization, coagulation, iron bacteria etc., where an adequate understanding of the chemistry of aqueous iron is relevant. The complexity of the aqueous iron system frequently is not fully appreciated. Many of the anomalous properties of iron compounds which are described in the literature can be explained by considering, besides the solubility equilibria of Fe(OH)~ and Fe(OH)a , the many other possible equilibria : solubility, complex formation, redox and 'hydrolysis', that may exist in iron bearing waters. In addition, the kinetics of each of these equilibrium reactions must also be considered.Q.u.ite obviously, this discussion of the chemistry of aqueous iron cannot be all inclusive. The authors have concentrated only on those theoretical aspects which in their opinion appear to be relevant in the iron chemistry of natural waters. Furthermore, no attempt has been made to completely cover the literature on this subject. The Solubility of Ferrous and Ferric IronIron may exist in solution in either the divalent, ferrous, or the trivalent, ferric state. In oxygenated water ferrous iron is oxidized to the ferric iron. The rate of this oxidation has been shown to be primarily dependent upon the hydrogen ion concentration and the temperature of the solution. The amount of iron taken into, or retained in solution, is frequently greater for ferrous than for ferric iron ; in other words, ferrous constituents tend to display a geater solubility than do ferric constituents.