The selective coadsorption of gaseous p-xylene and m-xylene is studied at 150 degrees C on BaY with a water content which varies up to 3.5% in weight. The preadsorbed water seems to be located only in the alpha-cages, probably around the compensation cations in sites II. The water, which is the more strongly adsorbed component, is not replaced during the coadsorption of xylene＇s, and this leads to a weakening of the interactions between the aromatic ring and the compensation cation. As for the anhydrous zeolite, two selective adsorption processes are discerned according to the filling. At low filling, BaY adsorbs preferentially the more abundant isomer in the adsorptive mixture, and the presence of water has no significant effect on the selectivity except for mixtures rich in p-xylene with which the selectivity is up to three time as great as that with the anhydrous zeolite. At high filling, BaY is selective for p-xylene whatever the composition of the adsorptive mixture, and the higher the water content, the higher is the selectivity for p-xylene. Obviously the enhancement of the selectivity caused by water preadsorption is to the detriment of the adsorption capacity of alpha-cages, which is decreased up to 20%.