A rather unusual behavior of the hydrogen evolution reaction characterized by the existence of two Tafelian regions at low and high overpotentials, respectively, is analyzed. These regions have the same or very similar slope, but different values of the current density extrapolated at the equilibrium potential. A kinetic interpretation of these systems, based on the Volmer-Heyrovsky-Tafel mechanism, is proposed. Experimental results are correlated and discussed in order to illustrate the derived expressions. (C) 2002 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.