The "hyper-reduced" [3Fe-4S](2-) cluster (consisting formally of three Fe(II) atoms and four sulfides (S2-)) possesses a remarkable capability for very rapid and reversible two-electron/two-proton oxidation (E-o＇ > -650 mV below pH 7) that is strongly suggestive of disulfide-based (as opposed to Fe-based) redox chemistry. This otherwise elusive reactivity is most readily revealed by performing fast-scan protein-film voltammetry on ferredoxins that contain a [3Fe-4S] cluster using an electrolyte composed of D2O. Fast, cooperative two-electron/two-proton transfer is observed after first generating the fully reduced [3Fe-4S](2-) state and then cycling rapidly to more oxidizing potentials. The unusual voltammetric characteristics can be modeled by using a coupled electron-transfer scheme involving multiple states of both the [3Fe-4S](0) and [3Fe-4S](0) forms. Rapid two-electron/two-proton oxidation produces an unstable species, most likely a disulfide, which may either be rapidly re-reduced or undergo an internal redox reaction to produce the normal [3Fe-4S](0) form, which formally comprises two Fe(III) and one Fe(II). Relaxation to the normal "0" form is a factor of 4 faster when the experiment is conducted in H2O, thus making the fast couple more difficult to observe-the retardation observed in D2O may be attributed to the need to rearrange the hydrogen-bonding interactions in the cluster binding domain.