Binary equilibrium data of selected C8 aromatics on Y zeolite and on silicalite were reported. These included the liquid phase excess adsorption isotherms at 30, 45 and 70°C, as well as the component adsorption isotherms of saturated vapor mixtures at 170°C. These isotherms were analyzed with the Real Adsorption Solution Theory (RAST). For the case of Y zeolite, the adsorption of a three component mixture could be closely predicted by the parameters obtained in the binary cases. In fact, the prediction of RAST differ little from the Ideal Adsorption Solution theory. The silicalite system was however highly nonideal. Although the RAST theory could reasonably fit the experimental data, the limiting activity coefficients so obtained reached as low as 10−4 or as high as 104 in several cases. This was clearly artificial. Furthermore, phase change of adsorbed solution was observed in the case of p-xylene ethylbenzene system. These observations called for a different model to explain the adsorption in silicalite.