The performance Of MoS2/Al2O3 catalysts, prepared by a sonochemical method and containing different amounts of fluorine, was investigated for hydrodesulfurization (HDS), using dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT) as model compounds. The activity of sonochemically synthesized MoS2 catalysts was higher than that of impregnated ones due to the improved dispersion of the Mo species. The addition of fluorine to the catalyst further increased the activity, reaching a maximum at the optimum fluorine content, due to an increase in acidity at the catalyst surface. The optimum amount of fluorine for maximum HDS activity was higher for the sonochemically prepared catalysts than for impregnated ones and, therefore, the activity of the former catalysts can be enhanced by fluorine addition to a greater extent than that of the latter types. The two promoting factors, sonochemical synthesis and fluorine addition, led to an increase in the hydrogenation (HYD) rates, compared to the direct-desulfurization (DDS) rates, in the HDS of both DBT and 4,6-DMDBT. However, the enhancement in overall activity was greater for the HDS of 4,6-DMDBT, which proceeds mainly via the HYD route, than for the HDS of DBT. The above reaction results, obtained using different catalysts, can be explained based on the surface properties of the catalysts, as characterized by X-ray photoelectron spectroscopy (XPS), nitric oxide chemisorption and infrared spectra of adsorbed pyridine. (C) 2004 Elsevier B.V. All rights reserved.