Grand Canonical Monte Carlo simulations are used to predict single component and binary mixture adsorption of p- and m-xylene in the ORTHO and PARA phases of silicalite. These predictions are compared with experimental results. Both phases adsorb p-xylene selectively, the ORTHO phase manifesting stronger p-selectivity than the PARA, p-Xylene is able to access the pore space of silicalite relatively easily while m-xylene is able to access the pore space only with difficulty; this seems to be the primary reason for p-selectivity. To contrast with situations where both isomers can access the pore space with relative ease, simulations of adsorption of xylene isomers in a larger pore structure are also performed. Under such conditions we observe that the ability of each component to form ordered clusters in the pore space influences selectivity. Oscillations in selectivity are observed and are attributed to competition between energetic and entropic contributions to adsorption. In all cases, sorbate-sorbate interaction between the components influences selectivity.