Monte Carlo adsorption simulations of xylenes have been performed in aluminophosphate molecular sieve structures. A new force field fitted for o-xylene in AlPO4-5 was used. It is shown that force fields have good transferability among the aluminophosphate sieves series and the new force field adequately describes the experimentally observed adsorption isotherms for xylene/AlPO4-5. A previous investigation of adsorption isotherms and structural analysis has been extended to AlPO4-8 and VPI-5 sieves. In AlPO4-8, like in AlPO4-5, the variations in the channels diameters and the corresponding interaction energy of the molecule-crystal lattice drive all molecular positioning. In VPI-5, the modulation between wide and narrow regions becomes negligible due to the larger pore diameter, so no ortho-selectivity was observed. The simulations confirm the ortho-selectivity mechanism proposed to aluminophosphates.1241795423434