Simulation studies are undertaken for the system Ag/Au(100) by means of grand canonical Monte Carlo applied to a large lattice system. The interactions are calculated using the embedded atom model. The formation of adsorbed AE phases of low dimensionality on Ag(100) is investigated and the influence of surface defects on the shape of the adsorption isotherms is studied. The results of the simulations are discussed in the light of experimental information available from electrochemical measurements.