Recently, it has been demonstrated that the [4Fe-4S] cluster of the Fe protein of nitrogenase from Azotobacter vinelandii can be reduced to an unprecedented all-ferrous state. In this work, the reduction potential for the formation of the all-ferrous state is measured by the reactions of the reduced and oxidized Fe protein with a variety of chemical redox active agents, and by mediated spectroelectrochemical titration. Redox titrations obtain a potential ca. -790 mV/NHE for the formation of the all-ferrous state, a value consistent with the chemical reactivity experiments and with recent theoretical calculations. At present, no known redox protein in A. vinelandii is capable of generating the all-ferrous Fe protein.