The transformation of thiophenic compounds (2-methylthiophene, 3-methylthiophene, 2-methylbenzothiophene and 3-methylbenzothiophene) was carried out in a fixed bed reactor at 350 degrees C under atmospheric pressure in the presence of a HY zeolite with a Si/Al ratio equal to 16. All the reactants underwent both isomerization and disproportionation. The product selectivities obtained experimentally were rationalized on the basis of the calculated stabilities (B3LYP method) of the expected cationic intermediates. These calculations allowed to explain why 2-methylthiophene and 3-methylthiophene presented the same reactivity in isomerization and why the methylbenzothiophene isomers were more reactive than the methylthiophene isomers. Moreover, the distribution of the dimethylthiophenes and dimethylbenzothiophenes obtained by disproportionation of the methylthiophene and methylbenzothiophene isomers, respectively, was in good agreement with the relative stabilities of the carbenium ions involved in these reactions. (C) 2007 Elsevier B.V. All rights reserved.