The adsorption properties of single-crystalline bimetallic surfaces differ from those of monometallic / elementary surfaces by the existence of a large number of different adsorption sites. While on smooth, ideal elementary surfaces the sites differ only by their coordination, i. e., by the number of surface atoms involved, such as adsorption on on-top sites (1 atom), bridge sites (2 atoms) or threefold sites (3 atoms), (smooth) bimetallic surfaces offer a large variety of sites or adsorption ensembles of different composition. For AB-type mixed surfaces, they may include A, B, AA, AB, BB, A3, A2B etc. sites.