Sorption of xylenes in two aluminophosphate molecular sieve structures, AlPO4-5 and AlPO4-11, has been studied using Monte Carlo simulations. Pure-component adsorption isotherms were predicted and compared with previously reported experimental data. Our simulated isotherms for xylenes in AlPO4-5 showed a qualitative agreement with the experimental data available especially the ortho-selectivity. However, for AlPO4-11, despite the qualitative agreement between the simulated and experimental isotherms, ortho-selectivity could not be precisely reproduced. A detailed structural analysis is also presented for each component in both molecular sieves. The analysis was performed using the detailed structures for the crystals and molecules, the statistics of the molecules centers of mass during the simulation, and the orientation of the normal vector to the plane of the molecules aromatic ring. Based oil this analysis, the adsorption sites and molecules positioning in the crystals channels were proposed. The molecules positioning confirmed the configurations proposed from experimental data by other authors. The simulations also showed that the ortho-selectivity previously observed in experimental results could be related to the variations in the channels diameters and the corresponding interaction energy of the molecule-crystal lattice. (c) 2005 Elsevier Inc. All rights reserved.8841699135144