Monte Carlo simulation is used to calculate the surface segregation in Cu-Ni and Pt-Ni alloys. The first system is characterized by its segregating tendency with a weak size effect, the second one exhibits an ordering tendency with a strong mismatch. Introduction of an elastic energy term in the Markov chain is necessary for the Pt-Ni system whose segregating properties are dominated by competition between chemical and elastic energy contributions. Interesting information is deduced from this kind of simulation concerning the atomic distributions in the surface plane.