A microscopic electronic theory is presented for the study of surface segregation of transition metal alloys. The total energy is written as a sum of attractive band structure energy and repulsive Born-Mayer-type interaction. The size-mismatch effect is treated within this electronic model instead of being introduced as a parameter. Through minimization of the surface free energy, the equilibrium concentration profile and surface relaxation of different faces of AuCu 3 are obtained as a function of temperature and compared with experimental data by low energy ion scattering (LEIS).