Adsorption isotherms and isosteric:heats of adsorption for benzene and p-xylene in silicalite have been calculated from molecular simulations. The simulations were performed using newly developed grand canonical ensemble Monte Carlo (GCMC) techniques in which insertion attempts are biased toward the most favorable regions of the zeolite pore space. The new techniques result in a substantial improvement in the efficiency of the simulations compared to traditional GCMC. The adsorption thermodynamics and molecular-level structure were studied for benzene and p-xylene in silicalite with Pnma symmetry (ORTHO) and P2(1)2(1)2(1) symmetry (PARA). The subtle differences between ORTHO and PAPA silicalite result in qualitatively different sorption behavior. An explanation of the experimentally observed step in the adsorption isotherm is presented, based on the results of the simulations and the ORTHO to PARA framework transformation that is observed experimentally. Predictions of the adsorption isotherms, isosteric heats, and siting locations of the adsorbates are in good agreement with experiment.