The mechanism of the first step (monosubstitution) of the liquid phase alkylthiolation of phenol by dimethyldisulfide over a dealuminated HY zeolite (Si/Al=20) as catalyst has been investigated. The kinetic study of the reaction suggests a Langmuir-Hinshelwood mechanism with a non-equivalent adsorption of the two reactants, leading to an inhibiting effect at high phenol concentrations. Infrared spectroscopic measurements of the adsorbed species confirm that phenol is strongly adsorbed on the catalyst by an interaction between the hydroxyl group of the phenol molecules and the strong acidic sites of the zeolite, while the adsorption of DMDS is much weaker. These studies allow to propose an overall mechanism, which takes into account the competitive adsorption of phenol and DMDS on different acidic sites of the zeolite, and which is in good agreement with the observed initial distribution of the two monosubstituted (alkylthio)phenols obtained.