To study the problems inherent in deep hydrodesulfurization (HDS), the separate and simultaneous HDS of 4,6-dimethyldibenzothiophene and hydrodenitrogenation (HDN) of pyridine were investigated over a Ni-MoS2/gamma-Al2O3 and a Pd/gamma-Al2O3, catalyst. The HDS of 4,6-dimethyldibenzothiophene and its three intermediates, 4,6-dimethyl-tetrahydro-dibenzothiophene. 4.6-dimethyl-hexthydro-dibenzothiophene and 4,6-dimethyl-dodecahydro-dibenzothiophene, demonstrated that, over the Pd catalyst, the (de)hydrogenation reactions were relatively fast compared to the C-S bond breaking reactions. whereas the reverse was true over the metal sulfide catalyst, The methyl groups of 4,6-dimethyldibenzothiophene strongly hinder the direct desulfurization HDS pathway over both catalysts. On the Pd catalyst the hydrogenation pathway is strongly promoted by the methyl groups, so that the total HDS rate does not decrease. Pyridine and piperidine were strong poisons, for the hydrogenation pathway and H2S was a strong poison for the direct desulfurization pathway. HDN of nitrogen-containing aromatic molecules occurs by hydrogenation of the aromatic heterocycle followed by C-N bond breaking. The C-N bond breaks by substitution of the alkylamine by H2S to form an alkanethiol, followed by the loss of H2S by elimination or hydrogenolysis. The NH2-SH substitution does not occur by a classic organic substitution reaction but through a multi-step reaction pathway via an alkylimine. DFT calculations showed that the hydrogenolysis of ethanethiol to ethane and the elimination of ethane are relatively easy reaction. (c) 2005 Elsevier B.V. All rights reserved.