More complex is the situation in the case of bimetallic surfaces, including metal film covered surfaces or surface alloys, or generally for multi-component surfaces. Staying at the case of bimetallic surfaces, or specifically, a metal substrate surface covered by an ultrathin film of another metal, the calculated energy difference between the slab and the bulk energies of the two components now involves not only averaging between the different surfaces, but also the energy required or released upon formation of the internal interface between the two metals. For a more detailed understanding and a precise determination of the various energy contributions, these have to be determined separately. Comparable problems also exist for the determination of the formation enthalpy. This is topic of the present publication, where, 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 report results of a theoretical study of the first principles determination of the surface energy and formation energy of different Ag/Pt(111) and PtAg/Pt(111) surfaces. In addition, for a better understanding of the underlying electronic effects, we will compare the resulting trends with those obtained for similar Ag/Pd(111) and PdAg/Pd(111) surfaces. Pt and Pd are well known for their close similarity in their structural, electronic, and chemical properties, e.g., in lattice constant or number of d-electrons. Nevertheless, there are distinct differences in their surface properties, such as alloy formation, the tendency for 2D phase separation in PtAg/Pt(111) while this is absent in PdAg/Pd(111), adsorption behavior,; pronounced site preference in CO adsorption on Pd(111), small differences in Pt(111), or surface diffusion of Had or COad .