The kinetic effect of quinoline (Q) and its reaction by-products, i.e. 1,2,3,4-tetrahydroquinoline (1,4-THQ), decahydroquinoline (DHQ) and orthopropylaniline (OPA), on the hydrodesulfurization (HDS) of 4,6-dimethyldibenzothiophene (4,6-DMDBT) was studied in the presence of a commercial NiMoP/Al2O3 catalyst under operating conditions relevant to industrial applications. Kinetic observations and modelling based on a Langmuir-Hinshelwood-Hougen-Watson (LHHW) mechanism, accounting for most of the observed Q and 4,6-DMDBT by-products, indicated that Q 1,4-THQ and DHQ are the main HDS reaction inhibitors, even at nitrogen concentrations as low as 25 ppm. To some extent Q and 1,4-THQ were adsorbed favorably on the HDS catalytic active sites, being more reactive than most of the sulfur and nitrogen-containing molecules. Besides, DHQ was less reactive than Q and 1,4-THQ despite a significant adsorption affinity on the HDS catalytic active sites. As a consequence. Q and 1,4-THQ inhibited HDS reactions at lower Q conversions, whereas DHQ inhibited them at higher Q conversions. (C) 2012 Elsevier B.V. All rights reserved.