This paper describes the identification of nitrogen-containing compounds in a typical feed for diesel oil hydrotreating and how their individual concentrations change upon hydrotreating over a conventional sulfided CoMo/Al2O3 catalyst at commercial conditions. A preconcentration procedure followed by gas chromatographic (GC) analysis utilizing a highly sensitive nitrogen-specific detector (atomic emission detector) allowed the quantitative analysis of individual nitrogen-containing compounds (N compounds) at levels as low as 0.05 mu g N/mL. The nitrogen compounds in the feed and products were identified by comparison with reference compounds as well as by high-resolution GC/mass spectrometric characterization. The relative reactivities of individual compounds in the diesel fuel feed were determined and the most refractory compounds identified. Alkyl-substituted carbazoles were found to be the major compound class in the feed and to be among the least reactive N compounds in the feed. Just as in the case of alkyldibenzothiophene hydrodesulfurization, carbazoles having alkyl substituents at positions adjacent to the nitrogen atom were found to be the least reactive N compounds in the diesel fuel feed for hydrodenitrogenation.