Adsorption equilibrium of benzene-p-xylene vapor mixture on silicalite is measured at 70-degrees-C with a specially designed cyclic volumetric apparatus at pressure levels of 2.53 and 1.20 kPa. The isobaric isotherms are S-shaped, and selectivity curves at different pressures cross over; the p-xylene selectivity at 2.53 kPa is higher than that al 1.20 kPa over a certain composition range. The heterogeneous ideal adsorbed solution (HIAS) model implemented on two patches qualitatively predicts these highly unusual behavior. The observed extraordinary phenomena are attributed to structural heterogeneity, a result of the tight-fit of sorbate molecules in silicalite pores. The success of HIAS attests to the paramount importance of adequate representation of heterogeneity in adsorption models.