Structural changes and catalytic performances of hydrodesulfurization (HDS) Co-Mo catalysts, supported on amorphous aluminosilicate (ASA) and amorphous SiO2 were investigated as a function of the amount of sodium ions added to the supports. The catalysts were prepared according to the incipient wet impregnation method using (NH4)(6)Mo7O24 as molybdenum precursor and CoCl2 or Co(NO3)(2) as cobalt precursor. Structural and morphological characterisations of the materials were performed with X-ray diffraction (XRD) and surface area measurements (BET). In the case of the ASA-supported catalysts, increasing the amount of sodium resulted in a gradual decrease of the catalyst surface areas and allowed formation of the beta-CoMoO4 phase. In the SiO2-supported catalysts, the beta-CoMoO4 phase formed in the absence of sodium. Moreover, addition of the alkali ions to the amorphous silica induced the phase transition to cristobalite resulting in a drastic decrease of the surface area. The catalytic behaviour of the materials was tested in the HDS of thiophene carried out in a continuous-flow system at atmospheric pressure, in a range of temperature between 603 K and 673 K. Unlike the silica-supported catalysts that were strongly inhibited by the addition of sodium, the ASA-supported catalysts exhibited a maximum of the HDS activity in correspondence with a specific amount of sodium. A correlation between structure and activity is proposed.