The inhibiting effect of basic and non-basic nitrogen compounds on the hydrodesulfurization (HDS) of dibenzothiophene (DBT), at conditions commonly used in the hydrotreatment of diesel feedstocks, has been experimentally determined. Nitrogen compounds studied herein are quinoline, indole, carbazole and a quinoline:indole:carbazole (Q:I:C) mixture in a proportion similar to that found in typical diesel feedstocks. The inhibiting effect of the non-basic compounds, indole and carbazole, was found to be comparable to that of the basic compound, quinoline. The inhibition behavior of these compounds is highly non-linear, showing strong inhibition at concentrations as low as 5 ppm las nitrogen). The Q:I:C mixture has a stronger inhibiting effect than any one of the individual components at equivalent concentrations. The experimental data for DBT HDS in the presence of quinoline, indole and carbazole was quantitatively represented by a modified Langmuir-Hinshelwood type rate equation and the data for the mixture by an extension of this model, using the apparent adsorption constants calculated from the inhibition results for the individual compounds.