A quasichemical formulation for chemical composition at the surfaces of nonregular solid solutions is presented and applied to Ag-Au and Cu-Ni alloys. The results are compared with the existing experimental values. It is observed that the use of the surface-energy data instead of the heat of vaporization gives good agreement with the experiments. Surface relaxation effects have been taken into account. The free energy of segregation is calculated for several layers. The surface short-range order parameters are found to be quite different from the bulk values. {\textcopyright} 1981 The American Physical Society.