The combination of acid and hydrodesulfurization catalytic functions on NiMo and CoMo catalysts supported on B2O3-Al2O3 (0-0, 1.0 and 3 wt% B) and its effect on the sulfur removal of 4,6-DMDBT were studied. The results show that boron-modified catalysts facilitate sulfur removal by promoting alternative catalyst functionalities, isomerization and hydrogenation, and that cooperation between these functions is required not only to generate intermediates easy to desulfurize but also to prevent catalyst deactivation. At 573 K, the reaction results indicate that acidity of boron-modified CoMo/Al2O3 is higher than the NiMo counterpart. So, CoMo/B2O3-Al2O3 displays higher hydrocracking of the n-hexadecane solvent and more isomerization products than those obtained with the NiMo catalyst. Maximum hydrodesulfurization activity of CoMo catalysts was obtained at boron contents of 1.0 wt% B. However, CoMo/B2O3-Al2O3 catalysts are susceptible to deactivation due to its strong acidity. NiMo catalysts are more resistant to poisoning by coke and show the highest HDS activity at 3.0 wt% B. The acid function on boron-modified catalysts was prone to deactivation at high temperatures or low hydrogen/hydrocarbon ratios such as those existing in batch laboratory reactors. (c) 2005 Elsevier B.V. All rights reserved.