Metal-based core-shell catalysts consist of a thin mono-or multilayer metal film covering a different metal core. Selectively changing the shell thickness, the particle size, or the composition of the core (alloying) or the shell results in changes in the electronic properties of the surface atoms, which in turn has a significant impact on their catalytic activity. Prominent examples are Pt-modified Ru-core nanoparticle catalysts, which were intensively studied over the last decade for the oxygen reduction reaction (ORR), the hydrogen oxidation reaction (HOR) including also the CO tolerance in that reaction, the hydrogen evolution reaction (HER), the formic acid oxidation reaction (FAOR), the ethanol oxidation reaction (EOR), the carbon monoxide oxidation reaction (COOR), or the methanol oxidation reaction (MOR). An important aspect for possible applications of such electrodes is their stability during operation. For Pt-Ru nanoparticles, this includes, in particular, the dissolution of the individual components into the electrolyte, where Ru dissolution is more facile than Pt, but also a possible surface restructuring. For Ru-core Ptshell nanoparticles, it was shown that the Pt shell slows down or even inhibits the dissolution of Ru, and the effect is usually stronger with increasing Pt layer thickness. Nevertheless, Ru dissolution often occurred also for these materials during long-term stability tests, which was attributed to the formation of pinholes in the Pt shell. Fundamental studies aiming at a more detailed understanding of the reactivity, adsorption properties, and stability of Ru-Pt coreshell electrodes often use Pt-modified Ru(0001) electrodes, which are perfect, well-defined model systems and therefore well suited for fundamental studies, including comparison of theory and experiment. Model studies using such electrodes in the ORR, the hydrogen underpotential deposition (HUPD), or the MOR revealed a distinct influence of the Pt layer thickness on Ru(0001). The stability of such model electrodes has been inves-tigated so far only for oxidation reactions on (sub-) monolayer Pt film-covered surfaces, employing ultrahigh vacuum (UHV) scanning tunnelling microscopy (STM) measurements before and after the electrochemical/-catalytic measurements. The results indicated that Pt protects the low-coordinated Ru atoms at step edges against dissolution, shifting the onset potential for dissolution to more positive values. For potentials larger than 1.05 V, Ru dissolution on the Pt-modified Ru(0001) electrodes was mainly observed at the Ru steps (step flow corrosion). Furthermore, Pt was found to restructure, forming Pt clusters on the surface in the regions of the original Pt islands.