x/Pt(111), these data demonstrate that ensemble effects do not play a significant role in these systems , as the CO adsorption energy hardly changes for CO adsorption on Pt1 monomers, Pt2 dimers or compact Pt3 trimers. Second, also COad coverage effects seem to have little effect. Changes in the CO adsorption energy with increasing coverages, up to a local coverage of CO = 1, are very small, at least for small Ptn ensembles. Hence, the effective COad -COad interactions must be small. This agrees perfectly with previous experimental findings from temperature programmed desorption experiments where peak broadening with increasing coverage was rather small, and also from equilibrium measurements. Third, the fact that CO adsorption on these pseudomorphic Ptn ensembles embedded in a matrix of Ag surface atoms is significantly stronger than adsorption on Pt(111), indicates that adsorption on Pt atoms in the surface alloy is dominated by lateral ligand effects rather than strain effects. In the present case of a pseudomorphic overlayer, we expect compressive strain induced by the larger Ag surface atoms, which would lead to a destabilization of the Pt-CO bond. Considering the increase in bond strength when going from CO adsorption on Pt(111)