The effect of a basic nitrogen compound (acridine) and of its main hydrogenation product (octahydroacridine) on the hydrodesulfurization of 4,6-dimethyldibenzothiophene (46DMDBT) was studied over a commercial NiMoP/Al2O3 catalyst under conditions commonly used in the deep hydrotreating of diesel fuels (340 degreesC; total pressure between 2.5 and 5.5 MPa). The sulfur and nitrogen compounds were in proportions representative of those found in diesel fuels (0.0014 MPa of 46DMDBT, 0.058 MPa of H2S generated from dimethyldisulfide and 0-0.0016 MPa of nitrogen compound in n-heptane). A significant negative effect of acridine on the hydrodesulfurization of 46DMDBT was observed whatever the total pressure (2.5-5.5 MPa) and the nitrogen content (0-200ppm). Both pathways, the so-called "hydrogenation pathway" (HYD) and "direct desulfurization pathway" (DDS) involved in the HDS of 46DMDBT, were strongly inhibited even with a very low concentration of nitrogen compound (20ppm N). However, after reaching a minimum, the activity through the DDS pathway increased with increasing acridine concentration and with increasing total pressure while the activity through the HYD pathway remained constant at its low level. It is suggested that this promotion effect on the DDS pathway is due to a cocatalytic contribution of acridine or of its hydrogenation products to the elimination process leading to C-S bond cleavage. Acridine was totally converted, mainly through hydrogenation, in the presence of 46DMDBT under the conditions of the experiment. Experiments with 1,2,3,4,5,6,7,8-octahydroacridine (OHA1), the major hydrogenation products of acridine suggest that this compound could be the actual inhibitor of the HDS of 46DMDBT. (C) 2004 Published by Elsevier B.V.