The inhibition effects of nitrogen species in gas oil on staged hydrodesulfurization (HDS) over commercial NiMoS/Al2O3 and CoMoS/Al2O3 catalysts of variable activity were investigated at 340 degreesC under initial 50 kg/cm(2) H-2 using as-distilled and nitrogen-free gas oils. Hydrogen renewal between stages was attempted to reveal an additional inhibition effects of the by-products such as H2S and NH3. All species in the gas oils were analyzed by GC-AED before and after hydrotreatment. NiMoS/Al2O3 and CoMoS/Al2O3 catalysts showed contrasting activities in HDS and susceptibility to nitrogen species according to their catalytic natures. HDS over NiMoS-2/Al2O3 of higher activity was largely improved by removal of nitrogen species, while that over NiMoS-1 catalyst of limited activity looked insensitive. Basically, HDS of reactive sulfur species suffered less inhibitions by nitrogen species than that of refractory species. In contrast, two CoMoS catalysts showed significant influences of nitrogen species. One of the most refractory species, 4,6-DMDBT, was effectively hydrodesulfurized by the removal of nitrogen species and renewal of hydrogen in the staged HDS, showing synergy effects due to simultaneous removal of two inhibitors. It must be noted that 107 ppm level of nitrogen species, basically carbazoles, which remained after the first stage HDS, influenced still the HDS of refractory sulfur species which is most expected in the second stage for the deep desulfurization. The acidity of the catalysts is compared to their inhibitions by nitrogen species as well as H2S and NH3 in the products. Additionally, the reactivity of nitrogen species in HDS is briefly discussed. (C) 2003 Elsevier B.V. All rights reserved.