Abstract : Number of alloys properties depends on the structure and the chemical composition of defects. In this context, we have studied interfacial segregation for a system presenting a strong tendency to bulk phase separate (e.g. Cu/Ag). This study has been performed by coupling Monte Carlo simulations with displacements and an effective Ising model. The considered interfaces are the Sigma=5 (310) <001> symmetrical tilt grain boundary as well as the (001) and (310) surfaces. For the grain boundary, Monte Carlo simulations show the existence of a 2-D compound in the grain boundary plane for an alloy with a strong tendency to phase separation in the bulk! For the (001) surface, we show the appearance of a crystallographical superstructure illustrating the complex coupling between segregation and structural change. A detailed matching of the Monte Carlo results on the rigid lattice model allows us to determine the interfacial segregation driving forces. The coupling of both methods highlights a significant contribution of the vibrational entropy to intergranular segregation, whereas it can be neglected for superficial segregation. A satisfactory evaluation of this entropic term must take into account for the vibrational coupling between nearest neighbors (thus the Einstein model cannot be used) and the differential thermal expansion near the interface. We then use the analytical approach to determine the segregation isotherms for the grain boundary. Thus, we show the existence of a first order multilayer phase transition. A rather similar behavior is obtained for the (310) surface, the interlayer interactions between open (310) planes being at the origin of these transitions both for the surface and the grain boundary. This multilayer aspect of intergranular segregation is then confirmed by Monte Carlo simulations. Moreover, these simulations show a wetting phenomenon, the initial grain boundary being split up into two interfaces separated by a metastable monocrystalline phase.