The effect of nitrogen-containing compounds (acridine and 1,4-dimethylcarbazole: 14DMCARB) on the hydrodesulfurization of 4,6-dimethyldibenzothiophene (46DMDBT) was studied on a sulfided NiMoP/Al2O3 catalyst in a fixed bed microreactor (340 A degrees C, 40 bar). Both nitrogen-containing compounds inhibited the hydrodesulfurization of 46DMDBT. However, the effect of acridine (a basic compound) and mainly its hydrogenated product (presumably 1,2,3,4,5,6,7,8-octahydroacridine: OHA1) was much more significant than the effect of 14DMCARB (a non-basic compound). In the presence of acridine, the so-called direct desulfurization pathway (DDS) of the HDS of 46DMDBT was less affected than the hydrogenation pathway (HYD) and was even slightly promoted when the partial pressure of acridine increased (after a strong initial inhibition). This was ascribed to a cocatalytic contribution of the nitrogen-containing compound to the C-S bond cleavage. 14DMCARB had the same inhibiting effect on both pathways (DDS and HYD). We also demonstrated that acridine inhibited the transformation of 14DMCARB and can explain why carbazoles are generally the main nitrogen impurities present in gasole after hydrotreatment.