For CO adsorption on PtAg/Ag(111) surface alloys with low Pd contents, prepared by Pt deposition at 380 K, Patel et al. reported a CO desorption peak at 380 K, which they attributed to CO adsorption on Pt1 monomers . At higher Pt surface contents (> 20%), a second peak at 477 K appeared, which the authors assigned to desorption from Pt ensembles. These desorption temperatures are significantly higher than those obtained for desorption from PdAg/Ag(111) surface alloys , which we mainly relate to the much more pronounced ensemble effect for CO adsorption on Pdn ensembles than for Ptn ensemble. While CO adsorption on threefold sites in Pd3 or larger ensembles is significantly stronger than on for on-top adsorption on Pd1 monomers, the differences are much smaller for adsorption on Ptn ensembles, with a small preference for CO adsorption on on-top (Figure ). All in all, the differences in CO adsorption on these two types of surface alloys, PdAg/Ag and PtAg/Ag , can be explained in a consistent picture, and the same is true also for the differences in CO adsorption when changing the substrate from Pd or Pt to Ag .