We report a laboratory and field study that demonstrates that soluble Fe(III) complexed to organic ligands exists in sediment porewaters. Synthetic solutions of Fe(III) with the ligand Tris were used to simulate organic complexation of Fe(III) in natural waters. Size fractionation using gel filtration chromatography shows that at least three different molecular mass species coexist. The molecular masses of FeIIIx−Trisy complexes increase with time, suggesting that aggregation occurs in solution. Soluble Fe(III) is detected by voltammetry at a mercury drop electrode when it is complexed by an organic ligand. Depending on the age of Fe(III), two voltammetric waves can be observed:  at ca. −0.3 V for “freshly” formed Fe(III) and at a more negative potential for “aged” Fe(III). A mathematical model was successfully developed to assess a mechanism for this aging process. This model involves oxidation and adsorption of Fe(II), aggregation, and precipitation of Fe(III) complexes. The species formed are extremely reactive. Upon addition of sulfide, reduction of Fe(III) instantaneously produces Fe(II) and FeS in solution, and a nonreductive breakdown of Fe(III) is promoted. The existence of soluble Fe(III) in sediment porewaters has important implications on the mineralization of organic matter. In addition to being highly reactive, Fe(III) can diffuse in and out of sediments, supplying an electron acceptor at locations where hydrous iron oxides are already consumed, potentially shifting local reactions. In addition, reduction of soluble Fe(III) by sulfide and successive formation of aqueous FeS should facilitate the production of pyrite in natural sediments. The use of voltammetry to measure soluble Fe(III) can be applied to any oceanographic or environmental system to determine the fate of Fe(III) in the dissolved phase.