Activation of RuO2 (110) and (100) single crystal surfaces by cathodic polarization was studied using hydrogen and oxygen evolution reactions as probes, as well as cyclic voltammetry and X-ray crystal-truncation-rod measurements. Both surfaces are poor catalysts for hydrogen reactions in the as-grown state, however, their activity increases after cathodic polarization. Cathodic activation of the RuO2 (110) surface was interpreted as the formation of metallic ruthenium sites which can be reoxidized to ruthenium dioxide. This process is reversible in the early stages of activation when no significant surface corrugation is produced. Irreversible surface roughening can be produced with activation at very negative potentials. On the other hand, the activation of the RuO2 (100) surface leads, under even very mild conditions, to an irreversible increase in surface pseudocapacitance and the appearance of new voltammetric features. This behavior is attributed to irreversible morphological changes following hydrogen intercalation into the subsurface region. The more facile and drastic changes upon activation of the (100) surface can be explained by its more open structure as compared with the densely packed (110) surface. (C) 2003 Elsevier Science B.V. All rights reserved.