An analytically solvable lattice model was used to study the difference in the physics of p-xylene adsorption in ORTHO and PAPA phases of silicalite. The model predicts that a phase transition of p-xylene in the pore space does not necessarily require a silicalite-phase transition. Conditions for the sorbate-phase transition are identified. The sign of the sorbate-sorbate interactions is critical to sorbate-phase transition. It is suggested that although the sorbate- and sorbent-phase transitions can be distinct events, there is strong coupling between the two. It is reasonable to suppose that the fluid-phase change may drive the zeolite phase change. Finally, this equilibrium study suggests the possibility of hysteresis at higher coverages depending on the kinetics of adsorption.