to CO adsorption on individual Pt surface atoms or small ensembles (Pt2, Pt3) in PtxAg1-x/Pt(111) surface alloys, ligand effects must be dominant in this system. In absolute terms, however, these changes are rather small. This fits well also to the trend observed experimentally, where the shifts in the desorption temperature with increasing COad coverage are rather small. Finally we compare these results with similar ones obtained for CO adsorption on PdxAg1-x/Pd(111) surface alloys. First of all, changes in the electronic properties of the Pd surface atoms with increasing Pdn ensemble size are very small, much smaller than for Ptn, as indicated by the small shift in d-band center. On the other hand, CO adsorption on PdAg/Pd(111) surface alloys shows much more pronounced shifts in the adsorption energy for increasing COad population of the Pdn ensembles, while changes in the calculated CO adsorption energies for the same site are small. The wide range in adsorption energies is reflected also by the considerable width of the CO desorption peaks in TPD measurements. The apparent discrepancy between these findings can be explained by pronounced ensemble effects on Pd-containing surfaces, which result in a significant increase in Pd-CO bond strength when changing from on-top adsorption on Pd1 monomers (-1.37 eV), which dominates at lowest Pd concentrations, to bridge site adsorption on Pd2 dimers (-1.64 eV), which becomes more dominant upon increasing Pd concentration, to finally adsorption on threefold hollow sites on compact Pt3 trimers and larger ensembles (-1.97 eV), which is the dominant site at higher Pd surface concentrations. This spread is in complete contrast to the negligible changes in adsorption energies observed on PtAg/Pt(111) monolayer surface alloys. Furthermore, for larger ensembles, also adsorbate -adsorbate repulsions play a role, while for small ensembles the displacement of adsorbed CO molecules by post-adsorbing CO molecules on less favorable adsorption sites, e.g., the replacement of a more strongly bound bridge-bonded COad on a Pd2 dimer by two less strongly bound COad on on-top sites, can become important. Such effects are much more pronounced for small Pdn ensembles than for small Ptn ensembles (see Figure ). The discrepancy between CO adsorption on these two monolayer surface alloys fits well to the trends observed for CO adsorption on Pd(111) and Pt(111):