Of the various properties of surfaces, chemical composi-tion is perhaps one of the most important that must be known in order to determine any other surface phenomena. Only re-cently, through the application of Auger electron spectrosco-py, has it been possible to analyze the chemical composition of the top-most layer at the surface in vacuum or at the solid gas interface. This nondestructive technique can provide qualitative and quantitative surface chemical analysis with a sensitivity of about 1% of a monolayer (about 10 “13 atom/cm2), and it is now possible to compare the composition of the surface with the known bulk composition. Simple thermodynamic arguments can convincingly demonstrate that the surface composition may be very different from the composition in the bulk for most multicomponent systems. Creation of a surface requires work, and it is always accompanied by a positive free energy change. Thus, in order to minimize the positive surface free energy, the sur-face will be enriched by the constituent which has the lowest surface free energy. This results, for many multicomponent systems, in gross imbalance between the surface composi-tion in the top-most layer and in the bulk. Even for monatomic solids, this surface thermodynamic driving force is the cause of the segregation of impurities at the surface that lowers the total surface free energy.

In many important surface phenomena, such as heteroge-neous catalysis or passivation of the surface by suitable pro-tective coatings, the chemical composition of the top-most layer controls the surface properties and not the composition in the bulk. It is therefore necessary to develop thermody-namic models that permit prediction of surface composition of multicomponent systems as a function of bulk composition and as a function of temperature. Thus, we would like to de-termine the surface phase diagram. In this paper we review the various thermodynamic models that permit practical determination of the surface composition of ideal or regular solid solutions. We shall summarize the ex-