Grand canonical Monte Carlo (GCMC) simulations were used to calculate energy contour maps, isodensity surfaces and adsorption isotherms of xenon in purely siliceous mordenite at 300 K using previously published potential parameters. Then, in order to understand the effect of aluminum content, Al-containing mordenites were modeled by adding the appropriate negative charge to each of the oxygen atoms in the mordenite framework and by inserting Na+ ions at fixed extra-framework positions previously identified from x-ray crystallographic data. GCMC simulations were again performed using these structure models. Both main channel and sidepocket xenon adsorption were observed in all mordenite structures. The sidepockets were found to be only singly occupied whereas the main channel allowed multiple occupancy. Sidepocket xenon adsorption became more favorable when cations (and aluminum) were introduced. In the main channel, xenon adsorbed into localized sites that were distributed in an octahedral arrangement within each unit cell with two apices in front of opposing sidepockets. These sites remained at the same location over the entire range of loading (0-16 Xe/unit cell) and cation (and aluminum) content.