The hydrodesulfurization (HDS) of 4,6-dimethyldibenzothiophene (4,6-DMDBT) was studied at 340 degreesC and 5 MPa in the presence of 2-methylpyridine and 2-methylpiperidine. Both N-containing molecules inhibited the HDS. The inhibitory effect of 2-methylpiperidine on the direct desulfurization and hydrogenation pathways of the HDS was slightly stronger than that of 2-methylpyridine. The desulfurization of 4,6-dimethyltetrahydrodibenzothiophene, an intermediate in the hydrogenation pathway, was extremely difficult in the presence of N-containing molecules. 4,6-DMDBT, in turn, inhibited the hydrogenation of 2-methylpyridine to 2-methylpiperidine but did not affect the C-N bond cleavage in the hydrodenitrogenation of 2-methylpiperidine. The use of toluene as an aromatic solvent had no effect on the HDS of dibenzothiophene and 4,6-DMDBT at 340 degreesC. Naphthalene inhibited the HDS of dibenzothiophene and 4,6-DMDBT without changing the product distributions. Both S-containing molecules suppressed the hydrogenation of naphthalene to the same extent. (C) 2004 Elsevier Inc. All rights reserved.