Organonitrogen compounds are present in commercial fuels after the hydrodenitrogenation process in quantities up to a few hundred ppmw N. Polyaromatic hydrocarbons (PAH) are present in significant quantities (1.4-11 wt %) in transportation fuels depending on both the region and season. In this work, the effect of organonitrogen ( quinoline, carbazole) and PAH ( naphthalene, fluorene, phenanthrene) compounds on desulfurization by pi-complexation with Cu(I) Y zeolite was studied and found to be moderate. The high selectivity for sulfur was not affected because a very low-sulfur fuel (< 0.1 ppm S) was produced prior to the sulfur breakthrough. The effects of these compounds were quantified by the decrease in desulfurization capacity of Cu( I) Y in the presence of each additive at a concentration of 1.6 mmol/L in a model fuel ( 100 ppmw S dibenzothiophene in 20 wt % benzene + 80 wt % n-octane). The extent of inhibition in desulfurization capacity by these compounds was found to follow the order carbazole > quinoline > phenanthrene > fluorene approximate to naphthalene. Ab initio molecular orbital calculations were also performed to provide an understanding of the inhibition effect. The observed order of inhibition was in good agreement with that predicted by molecular orbital calculation results.