The synthetic analog approach has been applied to a clarification of the active sites of 2Fe-2S* proteins. The compound (Et4N)2[FeS(SCH2)2C6H4]2, derived from o-xylyl-α,α′-dithiol, has been prepared and its structure has been determined by x-ray diffraction. The centrosymmetric anion contains two tetrahedrally coordinated ferric ions bridged by two sulfide ions and separated by 2.70 Å. Comparison of electronic, Mössbauer, and proton magnetic resonance spectra and magnetic susceptibility of the anion with the corresponding properties of the oxidized forms of the proteins reveals significant degrees of similarity. The anion also exhibits the essential redox capacity of the proteins. We conclude that [FeS(SCH2)2C6H4]22- possesses the same total oxidation level and electronic configuration as the active sites of the oxidized proteins, and that its structure provides a feasible representation of the minimal structure of the active site. [FeS(SCH2)2C6H4]22- is thus the first well-defined synthetic analog of the active sites of two-iron ferredoxins.