This study identifies and quantifies the key parameters in gasoil hydrodesulfurization at conversion levels above 95% (deep HDS) within the operating window of once-through trickle-flow units. The HDS reaction rate is markedly affected by trace amounts of basic organo-nitrogen compounds, viz. ca. 0-30 ppm, in the presence of >10 000 ppm polycyclic aromatics. We conclude that the dominant role of relatively high concentrations of basic organo-nitrogen compounds in model feeds reported in the literature closely resembles the presently identified key role of trace amounts in deep HDS of a very complex mixture such as gasoil. The order with respect to basic organo-nitrogen compounds in the HDS of a commercial gasoil is ca. -0.15, The influence of variations in the concentration of bulky polycyclic aromatics such as naphthalenes, chrysenes, and pyrenes is negligible. Furthermore, we employed a model gasoil to compare the activities of a NiMoP/Al2O3 and a CoMo/Al2O3 catalyst for the hydrodesulfurization of the refractory substituted dibenzothiophene compounds at the relatively low hydrogen-to-hydrogen sulfide ratio representative for once-through trickle-flow reactory, viz. ca. 15 Nl/Nl. In spite of this low hydrogen-to-hydrogen sulfide ratio, the NiMoP catalyst seems to display an unexpected activity advantage over the CoMo catalyst in deep HDS.