CoMo catalysts supported on amorphous aluminosilicates (ASA) with different Si/(Si +Al) ratios (0, 0.15, 0.25, 0.33, 0.5, and 0.75) were prepared and evaluated in the hydrotreatment of synthetic FCC naphtha. 2-Methylthiophene was used as the sulfur compound, and a mixture of 2,4,4-trimethyl-1-pentene and 2,4,4-trimethyl-2-pentene (approximately 3:1) was used as olefin. With the increase of Si/(Si + Al) ratio, the activity of hydrodesulfurization (HDS) and hydrogenation of olefin (HYDO) reactions performed in the CUS sites of Mo promoted by Co was inhibited. Meanwhile, increments in the acid-type reactions (alkylation of 2-MT with olefins and olefin isomerization and cracking) were observed. These changes were not gradual; a kind of transition zone with huge changes in product distribution between catalysts with 15 and 25% Si/(Si + Al) was observed. This was explained by an increment in the density and strength of Bronsted sites. These Bronsted sites compete with the CUS sites of Mo promoted by Co for reactant adsorption. Moreover, it was shown that skeletal isomerization could aid to control HYDO and cracking reactions. In general, it was demonstrated that by modifying the support composition, it is possible to modify the hydrodesulfurization, hydrogenation of olefins, and acid-type reaction activities. (C) 2010 Elsevier B.V. All rights reserved.