The evolution of H-2 from diamond cathodes according to the proposed mechanism H+(aq) + drop C-H + e(-) H-2 + drop C-. is treated with ab initio quantum mechanics. The solvated proton and diamond surface C-H bond are modeled with molecular clusters, and the electron is introduced at selected potentials by using a remote donor molecule. The reduction occurs when the electron affinity, EA, of the surface complex increases to the ionization potential, IF, of the donor as;he complex traverses its reaction coordinate. When they are equal, equilibrium is assumed and electron transfer occurs. The electrochemical potential, U, is given by U = (IP/eV - 4.6) V. For the H-2 generation reaction studied, the electron transfer coincides with the transition-state structure, and the activation energy is found to decrease as the potential becomes more negative. It is shown that surface C-H bonds will re-form by H+(aq) discharge on the surface carbon radicals. The potential-dependent reduction of H+(aq) to H-. is examined, too. Comparison is made with the gas-phase generation of hydrogen and surface carbon radicals. The present model employs the hydronium ion for H+(aq), methane and isobutane for surface C-H, and the self-consistent HF/STO-3G method. rn general, use of an electron donor is not required. This approach should find application to many other electrochemical reactions.