The selective separation of heavy n-paraffins (C-20-C-30) from refinery slack wax was performed by adsorption onto zeolite molecular sieves (5A). Sorbents were characterized by thermogravimetric analyses, N-2 adsorption, and mercury porosimetry. Three slack waxes: light, medium and heavy, were used, being analyzed by gas chromatography (GC) and simulated distillation. Several tests were carried out in a stirred batch reactor at high pressure and temperature for determining the system kinetics. Process variables were temperature (100-150degreesC), pressure (0.5, 1.5, and 2.5 MPa), and time (1-24 hr). Afterwards, adsorption isotherms in the range of 60-170degreesC were obtained for each wax by being fitted to a Langmuir type equation. Results showed. that adsorption equilibrium is controlled by internal diffusion due to the large length of the normal paraffin chains, which have a diameter very close to the size of the zeolite channels. The amount of paraffin retained in the sieve is strongly influenced by the feedstock: lighter products adsorb more, also producing a higher purity n-paraffin. The reverse trend is observed for the heavier feeds. However, adsorption of larger molecules (>n-C-32) is strongly hindered. For temperatures above 170degreesC, cracking reactions become more and more important.