The principal factors affecting aqueous chemical behavior of iron and manganese are shown in Eh-pH diagrams, designed to show predominant solute species, stable solid forms and solubility. Simple systems which contain water and the metal, and more complicated systems which contain fixed total amounts of one to four additional anions are considered. Relatively small shifts in Eh or pH can greatly change the equilibrium solubility of the metals. Some complexes, especially ferric fluoride and probably organic ferrous and ferric complexes, may strongly influence the solubility of iron. Because iron and manganese have low solubilities in oxidizing systems at pH levels near or little above neutrality, small shifts in Eh or pH, and complexing effects, may be very important in making the metals available to plants and animals.