The inhibition effect of carbazole (50-220 ppm as nitrogen) on the hydrodesulfurization (HDS) of dibenzothiophene (700 ppm as sulfur), at conditions commonly used in the hydrotreatment (HDT) of diesel feedstocks, has been experimentally determined. The inhibition behavior of this non-basic compound at this range of concentrations is non-linear, and was found to be weaker than the observed with other basic and non-basic nitrogen compounds also present in a diesel feedstock, like quinoline and indole, and than the one observed when carbazole is presented at lower concentrations (below 50 ppm as nitrogen). An inhibiting effect of dibenzothiophene over the carbazole hydrodenitrogenation (HDN) during the same process is also reported. Additionally, a retarding effect of carbazole over its own hydrodenitrogenation reaction during the dibenzothiophene hydrodesulfurization has been kinetically analyzed. The carbazole self-inhibiting effect was almost as strong as the observed for indole and ten times stronger than the observed for o-ethylaniline, according to the results reported elsewhere. The inhibitory effects described here were quantitatively represented by introducing apparent adsorption constants into the denominator of modified Langmuir-Hinshelwood (L-H) type rate equations. (C) 2004 Elsevier B.V. All rights reserved.