In this work, we focus on the stability of Pt-modified Ru(0001) electrodes with Pt thicknesses of up to three layers during the electrooxidation of methanol (MeOH) and CO in acid at potentials larger than 1.05 V. This potential had been chosen as upper potential limit in previous studies to prevent possible restructuring of the electrodes during the experiment. Here, we first studied the stability and structural modifications of an electrode with three Pt layers in pure supporting H 2 SO 4 electrolyte and during the MOR by surface X-ray diffraction (SXRD) at the synchrotron. The measurements indicated that these electrodes are surprisingly stable up to 1.4 V, in contrast to the submonolayer Pt modified Ru(0001) electrodes studied previously, or to pure Pt(111) electrodes. Inspired by this finding, we then explored the stability of Pt-modified Ru(0001) electrodes with a Pt thickness of one and three layers for the COOR. For these measurements, we chose the COOR instead of the MOR since