A strong synergistic effect of nickel and molybdenum was observed for hydrodesulfurization (HDS) of dibenzothiophene (DBT), 4-methyldibenzothiophene (4-MDBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT) over highly active Ni-MoS(2)/gamma-Al(2)O(3) catalysts. The surface of the most active catalyst in this study was composed by Ni:Mo = 1:3, which was determined by XPS results. In addition, catalytic behavior indicates the presence of two different adsorption models accounting for the DDS (direct desulfurization) and the HYD (hydrogenation) pathways. The structure of the NiMoS active phase appears to be identical regardless of Ni/(Ni + Mo) ratio. The reaction sequence of the DDS pathways is DBT > 4-MDBT > 4,6-DMDBT due to the steric hindrance of methyl groups located adjacent to the sulfur atom of DBT, which prevents sigma-bonding of the sulfur to the active site within the NiMoS phase. The reaction rates of HYD pathways are independent of the number of methyl groups, suggesting that the alkyl groups do not interfere the pi-adsorption of DBTs on the active sites. (C) 2010 Elsevier B. V. All rights reserved.