The removal evolution of six refractory individual S-compounds, i.e. DBT, 4-methyl-DBT, 4,6-dimethyl-DBT, 4-ethyl-6-methyl-DBT, 2,4,6-trimethyl-DBT, and 2,4,6,8-tetramethyl-DBT, and five groups of S-compounds with one to four substitutions has been studied during the deep desulfurization process of a heavy gasoil in a mini scale-structured bed reactor. The reactivity of the sulfur compounds was investigated by hydrotreating the gasoil with a commercial NiMo/gamma-Al2O3 catalyst under commercial operation conditions of temperature (563-623 K), total pressure (50 x 10(5) Pa), liquid hourly space velocity (LHSV) (0.7-3 h(-1)), and gas-to-liquid ratio (400-600 Nm(H2)(3)/m(oil)(3)). Moreover, the inhibiting effect of the hydrogen sulfide on the hydrodesulfurization rates of the S-compounds has been studied by performing tests with various H2S partial pressures. The results showed that the most refractory sulfur component is the 4,6-dimethyl-DBT constituting the major compound in the hydrotreated product with 50 ppm total sulfur and practically the only S-compound in the product with 10 ppm total sulfur. It has been verified that differences in HDS reactivity among DBT, 4-methyl-DBT, and 4,6-dimethyl-DBT in the real feed are lower than those commonly referred to in model feeds. The reactivity of the S-compounds is affected by the presence of the adsorbed on the catalyst active sites hydrogen sulfide and this effect appears decreasing with the number of the substituents of DBTs. The activation energy of DBTs obtained in the current work appears to be higher for the non-substituted DBT, which mainly reacts through the hydrogenolysis route, and to decrease for the substituted DBTs, which react through the hydrogenation route. (c) 2009 Elsevier Inc. All rights reserved.