In electrochemical promotion of catalysis (EPOC), the adsorption energies of reactants and products, and subsequently the overall reaction catalytic rate are modified by applying an electrochemical potential to the catalyst. In this paper, the oxidation of ethylene on ruthenium oxide was studied by experiments and theoretical modeling in order to elucidate the atomistic origin of EPOC. The experimental results have shown an increase in the reaction rate under negative and positive polarization. Density functional theory (DFT) based surface free energies demonstrated that there is an increase in oxygen coverage on the ruthenium surface as a function of the potential, conforming with the backspillover model of EPOC. Furthermore, DFT results demonstrated that the positive polarization and the associated electric field, which increases the work function, results in enhanced adsorption and facilitated cleavage of the CC bond of ethylene. Under negative polarization, on the other hand, it is the oxygen activation that is facilitated. Together, these two pieces of the puzzle explain the experimental increase of the ethylene oxidation rate as a function of positive and negative potential, proving the effect of an electric field on the adsorption rate and activation energy of ethylene oxidation.