For the surface alloys with two Ag sublayers, we again find an almost linear relation between the number of exchanged Pt surface atoms, but with a lower slope than for the PtAg/Pt4L surface alloys. In this case, the systems with x = 0 and x = 1 in Figure correspond to the Pt1L/Ag2L/Pt2L and Ag3L/Pt2L systems in Table , respectively. For a pure Ag surface layer (x = 1), all three surface alloys exhibit about the same formation enthalpy, which reflects the similar formation enthalpies of the AgnL/Pt(5-n)L systems with n=1 to n=3 (see Table ). Except for the first and second data point, the formation energies are smaller (less negative) than for the PtAg/Ag1L/Pt3L systems, and for all data points higher than for the PtAg/Pt4L surface alloys. The higher (more negative) values than for the PtAg/Pt4L surface alloys reflect a stronger bonding between Pt in the surface layer and Ag in the sublayer as compared to the mean value of the bulk energies (see eq. ( )). Apparently, this effect is more pronounced for the PtAg/Ag1L/Pt3L than for the PtAg/Ag2L/Pt2L system, which can be understood based on vertical ligand effects. Due to the stronger Ag-Pt bonding between the second and third layer (from the top) in PtAg/Ag1L/Pt3L compared to Ag-Ag bonding in the other system, bonding between the Ag subsurface layer and the surface layer is relatively stronger in the PtAg/Ag1L/Pt3L system. The lower slope compared to PtAg/Pt4L indicates that the energy gain per exchanged surface Pt, relative to the change in bulk energies, decreases less (more negative H) with increasing Pt content than for the PtAg/Pt4L system. For PtAg/Ag1L/Pt3L, this effect is apparently in between the other two cases, and not linear.