The properties of p-xylene and m-xylene adsorbed in NaY and KY faujasites 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 systems. This work is based on a new adsorbate-zeolite potential function named TrAZ potential (transferable adsorbate-zeolite potential). Adsorption isotherms and isosteric heats of adsorption of pure components have been calculated at 423 K and compared to the available experimental data. Grand canonical Monte Carlo simulations of coadsorption have also been performed. Three binary mixtures corresponding to different gas-phase compositions have been studied. The p-xylene/m-xylene selectivities predicted from the simulation are in good agreement with experiments: the NaY zeolite preferentially adsorbs the m-xylene isomer, whereas the KY faujasite is selective for p-xylene. From a detailed analysis of the xylene adsorption sites, we suggest a molecular mechanism of the cage filling in zeolites NaY and KY. Whatever the loading, our study reveals a unique adsorption site in the supercage of the zeolite NaY, located in front of the sodium cations in site II. In the case of the faujasite KY, three adsorption sites with different adsorbate-zeolite interaction energies are observed. From this microscopic analysis of the cage loading, the observed selectivities can be explained by joint energetic and steric effects.