The mechanism of discharge of a hydronium ion at a mercury electrode is explored using a combined microscopic approach. The quantum mechanical aspects of the proton and electron tunneling are addressed in the framework of Dogonadze-Kuznetsov-Levich theory. The cluster model is employed to describe the electrode surface- quantum chemical calculations are performed at the density functional theory level. The classical molecular dynamics simulations are performed for a hydronium ion at the mercury/water molecules interface. in order to judge about the distance dependent work term. The results of simulations indicate that the hydronium ion does not specifically adsorb at the uncharged electrode surface. The proton energy surfaces are built at several values of the H3O+-electrode distances using a quantum chemical approach, which allows for non-equilibrium solvent effects. The electron transfer is found to be adiabatic: the proton tunneling probability is estimated by means of an effective computational scheme. The Tafel plots are constructed in a wide range of the electrode potentials; their origin is ascribed mostly to the complex interplay between the partial contributions to the resulting current. An attempt is also made to model the kinetic isotope effect. (c) 2007 Elsevier B.V. All rights reserved.