Drastic regulations concerning sulfur content in fuels require the development of new processes in the refineries. In the case of gasoline, AOTS process based on the alkylation of thiophene by butenes was proposed. In the present work, we attempted to describe the mechanism of such a reaction with model molecules. Liquid-phase alkylation of 3-methylthiophene with 2-methyl-2-butene was performed on supported phosphoric acid. It is shown that this reaction is mainly selective to monoalkylation products. The kinetics is following an Eley-Rideal law and the reaction intermediate seems to be an ester of phosphoric acid or polyphosphate. (c) 2006 Published by Elsevier B.V.