The design of efficient hydrogen-evolving catalysts based on earth-abundant materials is important for developing alternative renewable energy sources. A series of four hydrogen-evolving cobalt dithiolene complexes in acetonitrile-water solvent is studied with computational methods. Co(mnt)(2) (mnt = maleonitrile-2,3-dithiolate) has been shown experimentally to be the least active electrocatalyst (i.e., to produce H-2 at the most negative potential) in this series, even though it has the most strongly electron-withdrawing substituents and the least negative Co-III/II reduction potential. The calculations provide an explanation for this anomalous behavior in terms of protonation of the sulfur atoms on the dithiolene ligands after the initial Co-III/II reduction. One fewer sulfur atom is protonated in the Co-II(mnt)(2) complex than in the other three complexes in the series. As a result, the subsequent Co-II/I reduction step occurs at the most negative potential for Co(mnt)(2). According to the proposed mechanism, the resulting Co-1 complex under-goes intramolecular proton transfer to form a catalytically active Co-III-hydride that can further react to produce H-2. Understanding the impact of ligand protonation on electrocatalytic activity is important for designing more effective electrocatalysts for solar devices.