A complete mechanism for the proton-dependent electrocatalytic reduction of CO2 to CO by fac-Re(bpy)(CO)(3)Cl that is consistent with experimental observations has been developed using first principles quantum chemistry. Calculated one-electron reduction potentials, nonaqueous pK(a)'s, reaction free energies, and reaction barrier heights provide deep insight into the complex mechanism for CO2 reduction as well as the origin of selectivity for this catalyst. Protonation and then reduction of a metastable Re-CO2 intermediate anion precedes Bronsted-acid-catalyzed C-O cleavage and then rapid release of CO at negative applied potentials. Conceptually understanding the mechanism of this rapid catalytic process provides a useful blueprint for future work in artificial photosynthesis.