After the potential cycling up to 1.35 V in CO saturated 0.5 M H 2 SO 4 (see electrocatalytic measurements below), the Pt-1 surface shows a distinct restructuring (Fig. ), in agreement with results in previous studies. This includes holes in the monolayer film and the Ru(0001) substrate at the former Ru step (marked by an arrow). Interestingly, the restructuring of the bilayer Pt film areas (marked by dashed circles) seems much more pronounced than that of the monolayer areas, which will be discussed below. One possibility for hole formation is the dissolution of material in the electrolyte as ions or oxidized species, which is known to occur for both pure Pt and Ru during the positive-and/or negativegoing scan in a complete potential cycle, in particular in the potential region for surface oxide formation and reduction. This is reflected in Pourbaix diagrams, and dissolution of Pt and Ru was directly demonstrated also by inductively-coupled plasma mass spectrometry (ICP-MS) measurements. For Ru, we suggest that dissolution is the main process behind the observed structural changes. For Pt, it is not as clear, and we will show later that under the present conditions, dissolution is probably unlikely.