97 , CO adsorption on PdAg/Pd shows considerable coverage effects, when comparing, e,.g., adsorption of a single CO on a Pd2 with adsorption of two CO on that ensemble . This is also indicated by the appearance of additional lower temperature peaks in the TPD spectra . A more detailed inspections revealed, however, that the decrease in effective adsorption energy is mainly due to changes in the adsorption site rather than directly caused by COad -COad repulsions. This is illustrated in As indicated before, we will also briefly compare with CO adsorption on Pd67Ag33 bulk alloys . This surface differs from the PdAg/Pd surface alloy by its slightly larger lattice constant and by the presence of Ag also in deeper layers, leading to a modification in strain effects and in vertical ligand effects. TPD spectra recorded on CO covered Pd67Ag33(111) (exposure at about 270 K) revealed a single peak only, with a maximum at about 370 K . This closely resembles the observations obtained for CO desorption from PdAg/Pd(111) surface alloys with surface Pd contents between 7% and 10%, which was attributed to desorption from Pd2 ensembles . The Pd content of the Pd67Ag33 surface in that study was estimated to be around 10% , which is close to the composition of the corresponding PdAg surface layer in the PdAg/Pd(111) sample. In a later atomic resolution STM study of a similar sample, Wouda et al. determined a Pd surface content of about 5%.