To investigate the effects of CO-substitution on the thermodynamics and the kinetics of proton binding in benzenedithiolate-bridged diiron complexes, the monosubstituted derivative Fe2(bdt)(CO)5(P(OMe)3) (2) was prepared by a CO-displacement reaction catalyzed by electron transfer. Kinetic analysis, based on cyclic voltammetry at various scan rates and ligand concentrations, indicates that the substitution reaction follows an associative mechanism. As the hexacarbonyl parent 1, the monosubstituted complex 2 neither reacts with protons in solution, nor catalyzes the proton reduction when acetic acid is used as a proton source. In the presence of a stronger acid, such as toluenesulfonic acid, voltammetry of 2 exhibits a reduction peak, which does not have a counterpart in the voltammetry of 1. We tentatively ascribed this reduction peak to the occurrence of a CE process involving proton binding by 2 and the reduction of 2H+ in the vicinity of the electrode. Bulk electrolysis experiments indicated that the proton catalysis is rather slow, i.e. few turnovers per hour. However, the onset of proton reduction occurs at about −0.8 V vs. Fc+/0 in MeCN + Bu4NPF6 electrolyte.