Finally we want to illustrate the role of ensemble effects, using the adsorption of hydrogen on PdAg/Pd(111) monolayer surface alloys as example. This differs from the situation experienced for CO adsorption in two aspects. First, stable adsorption on metals at room temperature is only possible for Interestingly, these three peaks are populated simultaneously during increasing hydrogen exposure (Figure ), which is in contrast to the normal behavior where the most stable sites, equivalent with the highest temperature peak, is filled first. This can be simply explained in a picture where surface diffusion of Had species from one Ptn ensemble to another one is inhibited. Thus, the different Ptn ensembles are populated simultaneously, e.g., by dissociative adsorption of surface diffusing H2,ad precursor species. This perfectly confirms our above assumption that always pairs of H ad have to be accommodated per adsorption of H2. A similar behavior was obtained also for H2 adsorption on PdAu/Pd surface alloys [141], further confirming these mechanistic ideas. Finally, it should be noted that according to the Sabatier principle (see section 3.2) the existence of three different H adsorption sites with very different binding energies on these surfaces offers a variety of different active sites, e.g, in hydrogenation reactions.