In situ FTIR spectroscopic and electrochemical data and ex situ (emersion) electron diffraction (LEED and RHEED) and Auger spectroscopic data are presented on the structure and reactivity, with respect to the electro-oxidation of CO, of the Ru(0001) single-crystal surface in perchloric acid solution. In both the absence and the presence of adsorbed CO, the Ru(0001) electrode shows the potential-dependent formation of well-defined and ordered oxygen-containing adlayers. At low potentials (e.g., from -80 to +200 mV vs Ag/ AgCl), a (2 x 2)-O phase, which is unreactive toward CO oxidation, is formed, in agreement with UHV studies. Increasing the potential results in the formation of (3 x 1) and (1 x 1) phases at 410 and 1100 mV, respectively, with a concomitant increase in the reactivity of the surface toward CO oxidation. Both linear (COL) and three-fold-hollow (COH) binding CO adsorbates (bands at 2000-2040 and 1770-1800 cm(-1) respectively) were observed on the Ru(0001) electrode. The in situ FTIR data show that the adsorbed CO species remain in compact islands as CO oxidation proceeds, suggesting that the oxidation occurs at the boundaries between the COads and O-ads domains. At low CO coverages, reversible relaxation (at lower potentials) and compression (at higher potentials) of the COL adlayer were observed and rationalized in terms of the reduction and formation of surface O adlayers. The data obtained from the Ru(0001) electrode are in marked contrast to those observed on polycrystalline Ru, where only linear CO is observed.