After the test run of several months two kinds of commercial catalysts (NiMo/Al2O3 and CoMo/Al2O3) were examined in hydrodesulfurization (HDS) of straight run (SRGO) and nitrogen-removed gas oils, at 340 degreesC under 50 kg/cm(2) H-2. Hydrogen renewal between stages was attempted to show additional inhibition effects of the by-products such as H2S and NH3- Spent NiMo/Al2O3 and CoMo/Al2O3 catalysts showed contrasting activities in HDS and susceptibility to nitrogen species, according to their catalytic natures, compared to those of their virgin ones. HDS over spent NiMo/Al2O3 was significantly improved by removal of nitrogen species, while that over spent CoMo/Al2O3 was much improved by H2 refreshment. The activity for refractory sulfur species such as 4,6-dimethyldibenzothiophene was reduced more severely than that for the reactive sulfur species such as benzothiophenes over spent catalysts. The effects of both two-stage hydrodesulfurization and nitrogen-removal were markedly reduced over the spent NiMo when compared with those over virgin NiMo one. The acidity of the catalysts was correlated with the inhibition susceptibility by nitrogen species as well as H2S and NH3- Spent catalysts apparently lost their activity due to the carbon deposition, which covered the active sites more preferentially. The spent NiMo catalyst carried more deposited carbon with larger C/H ratio and nitrogen content. Higher acidity was found to be present on the NiMo catalyst, but this was greatly decreased by the carbon deposition. Additionally, the reactivity of nitrogen species in HDS was briefly discussed in relation to the acidity of the catalyst and its deactivation by carbon deposition. (C) 2004 Elsevier B.V. All rights reserved.