Coadsorption of the gases p-xylene and m-xylene on BaY and NaY was studied at 150 °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 R -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 R -1 ) to that for single m-xylene (3.6 molecules R -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 R-cages. For filling lower than 2 molecules R -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 R -1 as the last molecules of xylenes are adsorbed in the R-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.

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