The tetrathionate/thiosulfate interconversion is a two-electron process: S4O6 2– + 2 e– ↔ 2 S2O3 2–. Both transformations can support bacterial growth since S2O3 2– provides an energy source, while S4O6 2– serves as respiratory electron acceptor. Interest in the corresponding S2O3 2– oxidation also arises from its widespread use in volumetric analysis of oxidizing agents and bleach neutralization during water treatment. Here we report protein film electrochemistry that defines the reduction potential of the S4O6 2–/S2O3 2– couple. The relevant interconversion is not reversible at inert electrodes. However, facile reduction of S4O6 2– to S2O3 2– and the reverse reaction are catalyzed by enzymes of the thiosulfate dehydrogenase, TsdA, family adsorbed on graphite electrodes. Zero-current potentials measured with different enzymes, at three pH values, and multiple S4O6 2– and S2O3 2– concentrations together with the relevant Nernst equation resolved the tetrathionate/thiosulfate reduction potential as +198 ± 4 mV versus SHE. This potential lies in the ∼250 mV window encompassing previously reported values calculated from parameters including the free energy of formation. However, the value is considerably more positive than widely used in discussions of bacterial bioenergetics. As a consequence anaerobic respiration by tetrathionate reduction is likely to be more prevalent than presently thought in tetrathionate-containing environments such as marine sediments and the human gut.