The transformation of thiophenic compounds (2-methylthiophene; 3-methylthiophene; 2,3-dimethylthiophene; 2,5-dimethylthiophene; 2-methylbenzothiophene; 3-methylbenzothiophene) was carried out over HY zeolite at 350 degrees C under atmospheric pressure in a fixed bed reactor. The transformation of some of them was also carried out over H beta, HZSM5 and H-mordenite (HMOR). In the presence of HY, all the reactants underwent both isomerization and disproportionation in various proportions depending on their structure. HZSM5 catalyzed isomerization only and disproportionation was also very limited compared to isomerization with HP and with HMOR. HMOR deactivated so rapidly that it was not possible to measure its activity accurately. Isomerization occurred essentially through an acid-catalyzed step by step intramolecular mechanism. The disproportionation product distribution was explained on the basis of the relative stabilities of the intermediates involved in the reaction. As expected, disproportionation was more sensitive to catalyst deactivation than isomerization. (c) 2006 Elsevier B.V. All rights reserved.