The properties of p-xylene and m-xylene adsorbed in sodium Y faujasite have been studied by grand canonical Monte Carlo simulation. Biased particle insertions and deletions were implemented to allow the computation of equilibrium adsorption isotherms of such complex molecules. Single species and binary mixtures isotherms at 423 K were calculated and compared to the available experimental data. Three binary mixtures corresponding to different gas phase compositions were studied. The p-xylene/m-xylene selectivities predicted from the simulation are in qualitative agreement with experiments : the zeolite NaY preferentially adsorbs the m-xylene isomer. A detailed analysis of the adsorbed phase structure reveals a unique adsorption site in the supercage of the zeolite located in front of the sodium cations in site II.