Grand canonical Monte Carlo (GCMC) simulations employing a rigid framework are performed to explore the adsorption behaviors of some aromatics in the purely siliceous MCM-22 zeolite, ITQ-1. Benzene, toluene, m-xylene and a-xylene are separately simulated at 315 K, investigating a series of pressure from 0.1 to 14 kPa. The potential adsorbed sites of the studied molecules are determined from the mass clouds. The comparison of the different mass clouds indicates that as the kinetic diameters change, the distributions of the adsorbate molecules present regular alterations. The predicted adsorption isotherms of two kinds of adsorbates, including toluene and m-xylene are in good agreement with the experimental results. Furhermore, the activation energies of xylenes migrating through the IO-MR windows interconnecting the 12-MR supercages are determined. The energy profiles distinctively suggest that near the 10-MR windows, the potential barrier of o-xylene is significantly larger than that of m-xylene, which will block the diffusion of o-xylene into the interior of the zeolite. The theoretical calculations and experimental results show that at relatively low pressure and temperature, benzene, toluene and m-xylene can migrate through the IO-MR windows and reach the adsorption equilibria easily, while o-xylene can not overcome the potential barriers along the 12-MR supercages to enter the interior of the zeolite. These results can be well understood by the existence of micropores or cavities of two different sizes: (1) the narrow 10-MR channels, which only benzene and toluene (or smaller molecules) can enter; (2) the wider 12-MR supercages, in which benzene as well as toluene and m-xylene can penetrate; (3) the much lower uptake of o-xylene or bigger molecules can be ascribed to the difficulty of the entrances of micropores and cavities.