The noninnocent coordinatively saturated mononuclear metal-thiolate complex ReL3 (L = diphenylphosphinobenzenethiolate) serves as an electrocatalyst for hydrogen evolution or hydrogen oxidation dependent on the presence of acid or base and the applied potential. ReL3 reduces acids to H-2 in dichloromethane with an overpotential of 380 mV and a turnover frequency of 32 +/- 3 s(-1). The rate law displays a second-order dependence on acid concentration and a first-order dependence on catalyst concentration with an overall third-order rate constant (k) of 184 +/- 2 M-2 s(-1). Reactions with deuterated acid display a kinetic isotope effect of 9 +/- 1. In the presence of base, ReL3 oxidizes H-2 with a turnover frequency of 4 +/- 1 s(-1). The X-ray crystal structure of the monoprotonated species [Re(LH)L-2](+), an intermediate in both catalytic H-2 evolution and oxidation, has been determined. A ligand-centered mechanism, which does not require metal hydride intermediates, is suggested based on similarities to the redox-regulated, ligand-centered binding of ethylene to ReL3.