The influence of nitrogen compounds on the hydrodesulfurization (HDS) activities of a series of substituted dibenzothiophenes in light cycle oil (LCO) was studied over a NiMo/Al2O3 commercial catalyst. Three types of light cycle oil with nitrogen compounds of different concentrations and chemical natures were used as feed-an original fluid catalytic cracking light cycle oil (LCO), LCO with most of its basic nitrogen removed, and an ultra-low nitrogen LCO. Experiments were conducted in a fixed-bed microreactor at a total pressure of 70 atm, temperatures between 330 and 400 degreesC, and liquid hourly space velocities (LHSV) in the range of 1.0 to 3.5 h(-1). The inhibition effects of nitrogen compounds on the HDS reactivity of the three sulfur groups-total sulfur, hard sulfur, easy sulfur-and 14 specific mono-, di- and tri-alkyl substituted dibenzothiophenes were investigated. The results showed that the RDS rate significantly increased using ultra-low nitrogen LCO. Pseudo first-order rate constants were estimated for the 14 mono-, di- and tri-alkyl substituted dibenzothiophenes. The HDS rates could be classified into three groups based on the position of the substituents. It was found that 4 and 6 substituted dibenzothiophenes had the lowest HDS rates. The HDS rate of the 14 substituted dibenzothiophenes were all increased when the ultra-low nitrogen feed was used. The improvement was greater for 4 and 6 substituted dibenzothiophenes than for those with one of the substituents at either the 4 or 6 positions. This finding indicates that the hydrogenation route is more strongly suppressed than hydrogenolysis route by nitrogen compounds since the hydrogenation route is believed to be the predominant reaction pathway for 4 and 6 alkyl-substituted dibenzothiophenes. (C) 2003 Elsevier B.V. All rights reserved.