Coadsorption of the gases p-xylene and m-xylene on BaY and NaY was studied at 150 degrees C in the range of pressure 10(-2)-3 hPa. For BaY coadsorption isotherms and single-component adsorption isotherms are perfectly superimposed in the whole range of investigated pressure. The capacity of adsorption of BaY is 3 molecules alpha(-1) for each single component as well as for each mixture under 3 hPa. For NaY coadsorption isotherms lie between those of single p-xylene and m-xylene. The capacity of adsorption of NaY for mixtures is in proportion to the composition of the initial adsorptive mixture and varies from the capacity of adsorption for single p-xylene (3.3 molecules alpha(-1)) to that for single m-xylene (3.6 molecules alpha(-1)) under 3 hPa. The selectivity of the Y zeolite depends on the filling, the composition of the mixture, and the exchangeable cation. Two selective adsorption processes are discerned according to the filling of a-cages. For filling lower than 2 molecules alpha(-1), BaY and NaY exhibit the same behavior toward the coadsorption of p-xylene and m-xylene and adsorb preferentially the more abundant isomer in initial adsorptive mixture : the selectivity depends only on the composition of the mixture. For filling higher than 2 molecules alpha(-1) as the last molecules of xylenes are adsorbed in the alpha-cages, BaY is selective for p-xylene whereas NaY is selective for m-xylene whatever the composition of the adsorptive mixture : the selectivity depends on the exchangeable cation. The dependence of the composition of adsorbate on the selectivity shows a nonideal behavior of the adsorbate.