In the present surface alloys, Pt surface atoms in a Pt1Ag8/Pt(111) surface alloy would be the most simple example (see Figure ). For the first COad the on-top site with a vertical COad is the most stable adsorption site and configuration. Adding a second COad, the most stable configuration involves adsorption on two bridge sites or threefold-hollow sites opposite from each other on both sides of the active Pt atom. For adsorption on threefold-hollow sites this must include one fcc and one hcp site, respectively. In both cases the COad are significantly tilted, by about 17° with respect to the surface normal, to increase the separation between the two molecules, in particular between the two O atoms.