This study aims to gain a quantitative understanding of the poisoning effect of 3-ethylcarbazole (3ECBZ) on the hydrodesulfurization (HDS) of 4,6-diethyldibenzothiophene (46DEDBT) in a fixed bed of a sulfided CoMo/Al2O3-SiO2 catalyst. A nonequilibrium, oneparameter model reproduces experimental results and quantifies how the traveling sulfur wave, nitrogen wave, and catalyst surface interact inside the reactor as the poisoning process proceeds. While 3ECBZs adsorptivity is about nine times that of 46DEDBT, its heteroatom removal reactivity is far lower than that of 46DEDBT. The catalyst can adsorb 3ECBZ at a rate at least 17-fold faster than it can denitroglenate. Thus even a trace amount of feed nitrogen can substantially retard the HDS rate. The model extracts adsorption and surface reaction rate constants and identifies two dimensionless groups governing the poisoning process. Fundamental and pragmatic implications of the results are discussed. (C) 2004 Elsevier Inc. All rights reserved.