Adsorption in the zeolite silicalite from binary liquid mixtures of p-Xylene, m-xylene, and toluene was investigated using grand canonical Monte Carlo (GCMC) simulations. The results obtained agree well with experimental excess adsorption isotherms from the literature. The agreement is very good when the zeolite-is modeled using the PARA form of the silicalite structure, but the results obtained with the native ORTHO structure are in some cases even qualitatively wrong. This supports the previous suggestion that the structure of silicalite undergoes a transition from ORTHO to PARA upon adsorption of aromatic molecules'. Molecular-level details of the energetics and siting within the zeolite provide insights into the macroscopic behavior. The simulated single-component and binary results were used to test ideal adsorbed solution theory for these systems. (C) 2004 American Institute of Chemical Engineers.