Bimetallic surfaces and catalysts have attracted considerable interest over recent decades, both from fundamental reasons, but in particular because of the attractive catalytic properties of bimetallic catalysts, which were often found to exceed the performance of their constituents in terms of activity, selectivity and stability. The improved catalytic performance was attributed to an interplay of a number of different effects such as geometric ensemble effects, electronic ligand effects and electronic strain effects, where the first describes the influence of the size/configuration of active surface ensembles. Electronic ligand effects describe the effect of electronic modifications of the active site by different neighboring surface atoms. These as well as strain effects, which reflect the effect of lattice distortions as compared to the natural lattice of the respective surface layer on the electronic structure, can lead to changes in the adsorption energy and thus to significant modifications in the reaction kinetics. Finally, site-blocking effects, which reduce the number of active sites, have to be considered as well. Initially, these different effects were mainly derived from changes in the catalytic activity upon varying the concentration of the respective components. More direct studies of these effects, correlating structural and electronic modifications with catalytic properties, became feasible with the advent of modern high-resolution spectroscopies and microscopies and in particular of theoretical methods, in combination with the increasing knowledge in the reproducible preparation of structurally well-defined model surfaces / systems. As part of an extensive series of combined experimental and theoretical studies on the structure, stability, electronic properties and adsorption behavior of structurally well-defined bimetallic PdAg/Pd(111) and PtAg/Pt(111) surfaces, we here report results of a theoretical study on the stability of different Ag/Pt(111) and PtAg/Pt(111) surfaces, as indicated by the formation enthalpy and surface energies, on their electronic properties, and on the CO adsorption behavior.