TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Component: The zeolite is zeolite Y that is partially in acid form. The lanthanide is present in the form of an oxide and/or ion. The quantity of lanthanide, expressed as an element varies at 0.5-20 wt.% (preferably 1-15 wt.%). The lanthanide is lanthanum. The metal of group VIII B is present in the form of an oxide, ion, and/or metal. The metal of group VIII B, expressed as an element, ranges at 0.001-10 wt.%, preferably 0.1-5 wt.%. The metal of group VIII B comprises platinum or palladium. The catalytic composition comprises a binder comprising silica, alumina, and clay, in a weight proportion of 50:50-95:5. The lanthanide salt comprises nitrates, citrates, acetates, sulfates, or chlorides.Preferred Concentration: The molar ratio between silicon oxide and aluminum oxide is 3-400, preferably 5-50.Preferred Method: The treatment with a lanthanum compound and the treatment with a compound of a metal of group VIII B comprise ion exchange, or impregnation. The ion exchange and impregnation are effected using an aqueous solution of a lanthanide salt and an aqueous solution of a salt of a metal of group VIII B.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Component: The mixture containing aromatic compounds comprises thermal or catalytic conversion plants and fractions of mineral oils. The fractions are gasoline from pyrolysis, fractions coming from pyrolysis gasoline, or fractions coming from aromatic production plants. The aromatic products are toluene, ethyl benzene, xylenes, benzene, 9C aromatic compounds, and/or naphthalene derivatives. The mixtures containing the aromatic compounds also contain cyclic alkanes and linear and/or cyclic alkenes. The resulting fraction of linear alkanes mainly consists of ethane, propane, n-butane, and n-pentane.Preferred Method: The charges are mixed with heavier fractions, coming from fuel oil from steam cracking or light cycle oil from fluid bed catalytic cracking. The method is carried out in the presence of hydrogen at 5-200 bar (preferably 50-70 bar), at 150-550 degrees C (preferably 300-500 degrees C). The method is carried out in continuous, in a fixed or fluid bed reactor, in gas or partially liquid phase, at a WHSV of 0.1-20/hour (preferably 0.5-3/hours). EXAMPLE - Commercial zeolite Y (25 g) with a molar ratio silica/alumina of 5.5 and a sodium content as oxide of 4 %, and 500 ml of a 2 molar aqueous solution of ammonium nitrate were charged into a glass flask. The suspension was left at reflux temperature for 3 hours, under stirring. After this period the mixture was filtered and calcined at 550 degrees C in air for 5 hours, obtaining a zeolite Y in acid form. The solid thus obtained was charged into the glass flask and 500 ml of 0.2 molar solution of lanthanum, nitrate hexahydrate were added. The solution was left at reflux temperature for 3 hours, under stirring. At the end of this period, the suspension was filtered, and the filtrate was washed with distilled water and dried in an oven. The above operation was repeated 3 times, for a total of four exchanges with a solution of lanthanum nitrate. After the last exchange, the solid was dried in an oven, and calcined in a muffle at 550 degrees C under an airflow. A zeolite Y was obtained, exchanged with lanthanum, with a molar ratio of silica/alumina equal to 5.6, a molar ratio lanthanum oxide/alumina of 0.22, and a molar ratio of sodium oxide/alumina of 0.0096.