The structural properties and the hydrodesulfurization (HDS) activity of sodium doped and sodium free CoMo catalysts supported on amorphous aluminosilicates (ASA) were investigated as a function of different SiO2:Al2O3 ratios. The support yielding the most active catalyst, (66% alumina) doped with different amounts of sodium, was used for a series of similar catalysts in order to study the effect of the alkali ion on the catalytic performance. The supports were prepared by sol-gel method and the catalysts were prepared by incipient wet impregnation method. The structure and the surface of the various samples were investigated by X-ray diffraction (XRD), temperature-programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS). The catalytic behaviour was tested in the hydrodesulfurization of thiophene carried out in a continuous flow system at atmospheric pressure, in a range of temperature between 603 and 633 K. Changes of activity with the support composition were observed. The presence of sodium, modifying the Bronsted acidity of the supports, enhances such effect. Moreover, the increase of the activity with increasing amount of sodium was a clear indication of the promoting effect of the alkali ion. (C) 2003 Elsevier B.V. All rights reserved.