The invention relates to a catalyst for isomerizing C.sub.8 aromatic fractions, the catalyst containing a mordenite and at least one metal from group VIII of the periodic classification of elements (such as Pt or Pd), characterized in that the mordenite is such that its skeleton Si/Al atomic ratio is between 6 and 10.5, its sodium weight content is below 2000 ppm, its unit cell volume is between 2.73 and 2.78 nm.sup.3, its benzene adsorption capacity is between 4 and 10% based on the dry mordenite weight and its 1,3,5-trimethylbenzene adsorption capacity is between 0.5 and 2.5% by weight based on the dry mordenite weight. The invention also relates to the preparation of the mordenite.