Rates and mechanisms of important reactions in the cycling of electrons via the geochemical transformations of iron have been identified using Mössbauer spectrometry. The cycling of iron through various reservoirs (aquifer, soils, sediments, claystone) depends on high surface-area-to-volume ratios of Fe-bearing solids. The ability of Fe-bearing solids surfaces to interact chemically, through surface complexation, and ligand exchange mechanisms, with reductants such as FeII, and oxidants such as Se, U, Tc, Co, Eu, and O2 facilitates electron transfer as well as dissolution and precipitation. Various pathways have been assessed on the basis of laboratory experiments for application to natural and engineered systems. FeII in the structure of layered silicates, oxides (e.g., Fe3O4) and hydrous oxides, and sulfides, as well as FeII surface complexes, such as on clay mineral edges, are very efficient reductants from a thermodynamic as well as from a kinetic point of view.