In order to study how adsorption of CO molecules changes thesurface composition of AuPd alloys, we develop a theoretical methodology whichis able to take this effect into account. An Ising model based on density functionaltheory calculations is derived to define interatomic potentials that describe metal−metal, metal−CO, and CO−CO interactions. Then, through the use of MonteCarlo simulations within the semi-grand canonical ensemble, the effect ofadsorption-induced segregation for the AuPd(100) surface is well-reproduced fordifferent temperatures and CO pressures. Segregation isotherms identify a Pdsurface enrichment for low CO pressures, and CO surface saturation is reached atan intermediate coverage of $\theta$= 0.5 ML. Furthermore, Pd chains induced by anordering of adsorbed CO molecules appear at low temperature and intermediateCO pressures. These chains are the result of a competitive effect between CO−CO repulsions and metal−CO interactions.