AbstractThe thermodynamics and kinetics of heterogeneous electron transfer (ET) for Co‐substituted horse myoglobin (Co−Mb) and its derivatives with ammonia and imidazole as heme axial ligands were studied with cyclic voltammetry on a pyrolytic graphite electrode along with their ability to mediate the electro‐catalytic production of H₂. All the proteins experience a non‐diffusive electrochemical regime as electrode‐bound species. The adsorbed Co−Mb construct carries out the electro‐catalytic reduction of water protons to H₂ with a good efficiency under anaerobic conditions thus yielding a simple and tunable system for H₂ production. Replacement of H₂O as Co axial ligand by ammonia and imidazole significantly lowers the catalytic currents for H₃O⁺/H₂O reduction to H₂. The E°’ values of the Co(III)/Co(II) redox couple for all species are mainly determined by the enthalpic contribution. Differences were found in the kinetics of ET for the different protein adducts due to changes in the activation enthalpies. However, all species share the same distance of about 14 Å from the electrode surface to the Co(III)/Co(II) center determined using the Marcus model, consistent with a non‐denaturing adsorption of the protein.