In contrast to the above argumentation, however, we observed that very thin Pt films (incomplete Pt monolayer or bilayer films) are less stable than Pt(111). We suggest that the enhanced restructuring tendency of the (incomplete) monolayer film is induced by the strong interaction of OH/O species with the Ru(0001) substrate, which must be considered. Apparently, the gain in energy provided by the strong interaction of OH/O species with Ru(0001) is sufficient to displace Pt from monolayer sites into next layer sites, providing space for an additional uptake of OH/O species on the Ru(0001) substrate. Computational works showed that Pt atoms in the second layer bind less strongly on the first Pt layer than Pt atoms directly attached to the Ru(0001) substrate. However, this loss in energy is apparently overcompensated by the energy gain of the OH/O adspecies, if they are strongly adsorbed on Ru(0001) rather than weakly adsorbed on a Pt monolayer film. The energy gain of the entire system upon OH/O-induced displacement of Pt atoms into the second or higher layers provides the energetic driving force for such kind of restructuring. For the Pt-2 electrode, the COOR data indicate that these electrodes seem relatively stable when the layer is closed (up to 1.3 V, see Section S5 in the SI), which does not seem to be the case in the CO-free supporting electrolyte (Section S4 in the SI). Based on the explanations above, we would expect that Pt bilayers are less stable than the monolayer but also more stable than the third layer. The physical origin of this discrepancy is so far open.