A simple diiron dithiolate [(mu-S2C4N2H2)Fe-2(CO)(6)] (M2) as mimic of the active site of [FeFe] hydrogenase catalyzed the reduction of protons to dihydrogen (H-2). The strength of the acid played an important role in controlling the mechanism of electrocatalytic proton reduction. The catalysis of weak acid (acetic acid) occurred around -2 V us. Fc/Fc(+), at which M2 did not have a counterpart in the absence of acid. The catalyst was actually the reduced form of M2 and an (E)ECEC mechanism was proposed. With a moderately strong acid (trifluoroacetic acid), two significant catalyses operated. Reduction of the 2e(-) + 2H(+) intermediate ([M2IM-H-2]) catalyzed the proton reduction at ca. -1.4 V, whereas a competed catalysis at ca. -1.6 V dominated when the protonation of the reduced form of [M2IM-H-2] exceeded H-2 elimination at higher acid concentrations. Upon addition of a strong acid (triflic acid), one of the ring nitrogen atoms could be protonated. After two successive one electron reductions, the formed species proceeded a catalytic cycle via a CECE process at a comparatively mild potential of -0.65 V vs. NHE. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.