Cyclic voltammetry (CV) was used to investigate the electrocatalytic oxidation of formic acid on smooth and rough Ru(0001) electrodes in HClO4 solution. Ex-situ electron diffraction (LEED and RHEED) and Auger electron spectroscopy (AES) were applied to characterize the Ru electrode surfaces and the adsorbate structure. The CV for a smooth Ru(0001) surface in a 0.1 M HCOOH+0.1 M HClO4 solution no longer exhibits the H- and OH-adsorption peaks in the double layer region due to complete blocking of H/OH-adsorption by COad species, as has been confirmed by the observation of an ordered (2 x 2)-CO phase in good agreement with in-situ IR measurements. We find that the major pathway for HCOOH oxidation is via dehydration at electrode potentials below 0 V involving no faradaic current formation in agreement with the literature, since no anodic current peak occurs in this potential region. An anodic current peak appears at 0.6 V, which is ascribed to COad electrooxidation with concomitant O/OH adsorption as confirmed by a (1 x 1)-O phase and an increase in the Auger O-signal, which causes inhibition of HCOOH adsorption and decomposition leading to deceleration of formic acid oxidation at higher potentials, E > 0.7 V. (C) 2003 Elsevier Science B.V. All rights reserved.