The reduction of acids has been categorized according to reaction mechanism and a variety of acids from pK(a) 1.6 to 13.3 has been studied in dimethyl sulfoxide at platinum electrodes. Acids with pK(a) up to similar to 5 are reduced by a CE mechanism involving prior dissociation to form the solvated proton which in turn is reduced to dihydrogen. Measurements of equilibrium potentials and simulations of cyclic voltammograms are consistent with a formal potential for the H+ /H-2 couple of - 0.79 V versus the formal potential of ferrocene. Discrepancies between the simulations and experimental voltammograms have been interpreted as contributions from underpotential deposition of adsorbed hydrogen atoms, a process not included in the simulation model. Near pK(a) = 6, the dissociation of the acids becomes too slow to support the diffusion-controlled currents that are observed. Here a transition occurs and the mechanism changes to direct discharge of the acid with transfer of the hydrogen atom to the platinum surface. The results for six acids between pK(a) 6 and 13 are consistent with transition to this mechanism. The acid types included OH, NH and CH acids. Evidence has been found that one of the CH acids, ethyl nitroacetate, may undergo tautomerization to an OH form prior to direct discharge.