Grand canonical ensemble Monte Carlo simulations of the adsorption properties of several model faujasite zeolites were performed using the statistical bias method. The results enable a better understanding of the effect of cation exchange in the selective adsorption of binary mixtures of para- and meta-xylene isomers. The NaY/KY reversal of adsorption selectivity (from m- to p-xylene) can be accounted for by a simultaneous change in the size of the cation and a displacement of the potassium cation towards the centre of the supercage, this gives rise to a complete rearrangement of the adsorption potential energy surface. BaX is shown to be selective in p-xylene because of the sufficient size of the barium cation. Increasing the number of cations in the NaY faujasite model had a similar effect on selectivity to increasing the cation size. On the basis of our simulations, we predict that NaX should be only weakly selective in favour of one or the other xylene isomer. We also predict that adding a small amount of water molecules could enhance the adsorption selectivity in favour of p-xylene.