The influence of 2-methylpyridine and 2-methylpiperidine on the hydrodesulfurization of dibenzothiophene (DBT) and the effect of DBT on the hydrodenitrogenation of 2-methylpyridine and 2-methylpiperidine were studied over a sulfided NiMo/Al2O3 catalyst at 5 MPa, 35 kPa H2S, and 300 and 340 degreesC. Both N-containing molecules strongly suppressed the hydrogenation pathway of the hydrodesulfurization of DBT and inhibited the direct desulfurization route at both reaction temperatures. The inhibitory effect on the direct desulfurization was stronger for 2-methylpyridine than for 2-methylpiperidine. H2S promoted the hydrogenation of 2-methylpyridine up to 10 kPa and inhibited it at higher partial pressures. H2S had a positive influence on the hydrodenitrogenation conversions of 2-methylpiperidine and 2-methylpyridine. DBT had a negative effect on the hydrogenation of 2-methylpyridine, but did not influence the C-N bond cleavage of 2-methylpiperidine. Therefore, C-N and C-S bond breaking takes place at different active sites, whereas the hydrogenation sites for N- and S-containing molecules may be the same. (C) 2003 Elsevier Inc. All rights reserved.