Separation processes using selective adsorption on zeolite are widely applied in chemical industries. The X zeolite is especially used for the separation of xylenes isomers. The objective of this work-based on NaX adsorbent known by its selectivity exclusively geometric to xylene isomers-is a contribution to the comprehension of the separation mechanisms and of the steric effect parameters. Adsorption of o-xylene, m-xylene, p-xylene, and trimethylbenzene was studied in thermobalance at 100, 250, and 420 degreesC and at a constant pressure of 700 Pa. The results show that the adsorption and diffusion process are strongly influenced by the molecules' sizes. Using the Fick law plot, disruption phenomena owing to the steric effect were observed when the supercages of NaX adsorbent were filled.