Ru(0001) electrodes modified by a single (Pt-1) or 3.5 monolayers (ML) of Pt (Pt-3) were prepared and characterized by STM imaging under UHV conditions, as described in the Experimental Section. Schematic illustrations of the surface cross-section and representative STM images of the as-prepared Pt-1 and Pt-3 electrodes are shown in Figs. and, respectively. As illustrated by these images, the Pt film does not grow in a perfect layer-by-layer fashion due to kinetic limitations. On the Pt-1 electrode, the film consists of monoand bilayer islands, exposing also the Ru(0001) substrate. The Pt-3 electrode consists of regions with two to four Pt layers and a few small five-layer islands. Figs. and show STM images of the electrode surfaces recorded under UHV conditions after exposing the electrodes to potential cycles up to 1.35 V during the COOR (see Fig. ).