Cyclic voltammetry experiments and DFT calculations allowed us to establish a complete mechanism of the catalysis of electrochemical proton reduction by [Fe-2(mu-SCH2N(H)CH2S)(CO)(6)] (Fe-adt) in acetonitrile. The proposed mechanism is fully consistent with the observed dependence of the voltammetric responses on the strength of the acid used as a proton source. Addition of moderately strong acids, such as CCl3CO2H (pK(a) = 10.7) or HOTs center dot H2O (pK(a) = 8.6), triggers the occurrence of new reduction events at potentials less negative than the reduction of Fe-adt, therefore ascribed to reduction of the protonated forms of the complex. Reduction of the N-protonated form seems to favor a tautomerization reaction leading to a Fe-H intermediate. On the other hand, addition of weak acids, such as ClCH2CO2H (pK(a) = 15.3), leads to direct protonation on the diiron site subsequently to reduction of the catalyst. A better understanding of the mechanism of proton reduction by the biologically relevant Fe-adt derivative could impact the design of improved catalysts inspired by FeFe-hydrogenase.