The p-xylene adsorption isotherms at 22 degrees C on a H-3.8-ZSM-5 material and two of its Cs-2.0- and Cs-3.8-ZSM-5 cesium exchanged phases show that the saturation capacities are 7.9(2), 6.0(2), and 2.9(2) sorbate molecules/unit-cell (uc), respectively. The crystal structures of two xylenates corresponding to the 4p-xylene loaded H-form and the totally Cs-exchanged and p-xylene saturated phases are determined by X-ray powder full profile refinements. Difference-Fourier maps clearly reveal the locations of C8H10 and the major Cs site. In H-3.8-ZSM-5 . 3C(8)H(10), p-xylene is located at the channel intersection (site I). In the Cs-3.8-ZSM-5 . 2.6C(8)H(10) phase, p-xylene is also located at site I and the Cs cations are spread over three distinct Cs1, Cs2, and Cs3 sites presenting 1.1(1), 2.0(1), and 1.0(1) cations/uc populations, respectively. Cs1 presents a twofold positional disorder and is located midway between sites I and II (the zigzag channels), the mean Cs1-O(framework)x6 contact being 3.4(1) Angstrom. Cs2 lies almost near the m plane at y = 0.285(2) (Pnma space group) and is in the pi-interaction with the aromatic ring, the mean Cs2-Cx6 distance being 3.0(1) Angstrom. Cs3, which is located at site Pi, also presents a twofold positional disorder and might be associated with residual water molecules, the mean Cs3-O(framework)x6 contact being 3.3(1) Angstrom.