Cobalt complexes supported by diglyoxime ligands of the type Co(dmgBF(2))(2)(CH3CN)(2) and Co(dpgBF(2))(2)(CH3CN)(2) (where dmgBF(2) is difluoroboryl-dimethylglyoxime and dpgBF(2) is difluoroboryl-diphenylglyoxime), as well as cobalt complexes with [14]-tetraene-N-4 (Tim) ligands of the type [Co(Tim(R))X-2](n+) (R = methyl or phenyl, X = Br or CH3CN; n = 1 with X = Br and n = 3 with X = CH3CN), have been observed to evolve H-2 electrocatalytically at potentials between -0.55 V and -0.20 V vs SCE in CH3CN. The complexes with more positive Co(II/I) redox potentials exhibited lower activity for H-2 production. For the complexes Co(dmgBF(2))(2)(CH3CN)(2), Co(dpgBF(2))(2)(CH3CN)(2), [Co(Tim(Me))Br-2]Br, and [Co(Tim(Me))(CH3CN)(2)](BPh4)(3), bulk electrolysis confirmed the catalytic nature of the process, with turnover numbers in excess of 5 and essentially quantitative faradaic yields for H-2 production. In contrast, the complexes [Co(Tim(Ph/Me))Br-2]Br and [Co(Tim(Ph/Me))(CH3CN)(2)](BPh4)(3) were less stable, and bulk electrolysis only produced faradaic yields for H-2 production of 20-25%. Cyclic voltammetry of Co(dmgBF(2))(2)(CH3CN)(2), [Co(Tim(Me))Br-2](+), and [Co(Tim(Me))(CH3CN)(2)](3+) in the presence of acid revealed redox waves consistent with the Co(III)-H/Co(II)-H couple, suggesting the presence of Co(III) hydride intermediates in the catalytic system. The potentials at which these Co complexes catalyzed H-2 evolution were close to the reported thermodynamic potentials for the production of H-2 from protons in CH3CN, with the smallest overpotential being 40 mV for Co(dmgBF(2))(2)(CH3CN)(2) determined by electrochemistry. Consistent with this small overpotential, Co(dmgBF(2))(2)(CH3CN)(2) was also able to oxidize H-2 in the presence of a suitable conjugate base. Digital simulations of the electrochemical data were used to study the mechanism of H-2 evolution catalysis, and these studies are discussed.