In this paper we discuss the mechanism of the electrochemical hydrogenation of aliphatic ketones at platinum electrodes in aqueous acidic electrolytes. Based on the potential dependence of the measured reaction orders and on kinetic considerations, we derive that hydrogen competes with ketones for adsorption sites. Hence, hydrogen underpotential deposition (H-upd) exerts an inhibiting influence on the reduction of ketones and eventually suppresses it. The variation of the proton concentration has a significant influence on the reaction rate of acetone, but not on the reaction rate of higher aliphatic ketones, from which it is derived that the active species of acetone reduction is the protonated acetone, which forms in solution in a pre-equilibrium. The absence of a proton effect for the hydrogenation of higher aliphatic ketones indicates that in those cases the enol tautomer is adsorbed to the surface. It is concluded that after reductive adsorption of protonated acetone to the electrode the reduction proceeds in the rate determining step via a proton-coupled electron transfer to the adsorbate. The adsorbate, in which the carbonyl functional group is transformed to an OH-group, is either immediately adsorbed to the surface from solution (acetone, protonated at the carbonyl oxygen) or formed by a surface reaction between the enol (higher aliphatic ketones) and adsorbed hydrogen. The presence of the enol and its reaction with adsorbed hydrogen to the protonated ketone was concluded from Surface-Enhanced Raman spectra. Adsorbed hydrogen is, therefore, necessary to provide the active species of ketone reduction, which is reduced in the rate determining step by the proton-coupled electron transfer. (C) 2018 The Authors. Published by Elsevier Inc. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).