Due to its structure and high Si/Al ratio, zeolite mordenite has high thermal and acidic stability. Mordenite-type of zeolites have been used as catalysts in many industrially important reactions such as hydrocracking, hydroisomerization, alkylation, acid-catalyzed isomerization of alkanes and aromatics, reforming. In order to overcome the problem of the limited access to the active sites, OSDA-free synthesized mordenite undergoes fluoride etching as a post-synthetic treatment. The post-synthetic treatment is performed with hydrofluoric acid in combination with ammonium fluoride. Thus, the porosity is enhanced additionally without changing considerably the Si/Al ratio of the zeolite framework. All samples have been characterized by X-ray diffraction analysis, nitrogen adsorption, scanning electron microscopy, high-resolution transmission electron microscopy and solid-state nuclear magnetic resonance spectroscopy. The catalytic activity of the samples obtained has been investigated in the reaction of m-xylene transformation. All mordenite samples having undergone post-synthetic treatment exhibit catalytic activity higher than that of the parent sample.