During the study of mordenite with the EPR technique, three EPR signals centered at g = 4.3; g = 2.4 and g = 1.98 were evidenced. The first two were attributed to Fe3+ ions (signal at g = 4.3 typical to isolated framework iron species, signal at g = 2.4 characteristic to agglomerated non-framework ones). The signal at g = 1.98 was assigned to Cr5+ ions. All these cations are present in the solid as impurities. Furthermore, under certain conditions the Cr5+ ions can be in interaction with two equivalent Al-27 nuclei according to a formation of a superhyperfine structure in the EPR signals. After adsorption of naphtalene on the mordenite samples, organic pi-radicals are formed via secondary reactions of primary cation-radicals nearby the aluminum atoms of the mordenite framework. The formation of radicals seems to be correlated to the presence of impurities in the solids. Thus not only acidity but also content of transition metals may be an important factor in the formation of radicals.