Hydrotreatment of SRC-II heavy distillate was investigated over presulfided CoMo/Al//2O//3 catalyst at 390 degree C, 10MPa both in H//2 flow and in 5%H//2S/H//2 flow. Elemental analysis, **1H-NMR, FT-IR and column chromatography were used for characterization of product oils. It was observed that hydrotreatment over presulfided catalyst decreased both nitrogen and sulfur contents. It was also found that gas phase addition of H//2S promoted HDN reaction and inhibited HDS reaction. In order to clarify the promoting effect of H//2S on HDN reaction, product oils were separated into base components by column chromatography and investigated in detail. Hydrotreatment in H//2S/H//2 flow removed 80% of base component of reactant. Comparing with the reaction in H//2 flow, removal of strong base was especially promoted in the presence of H//2S/H//2 and removal of very weak base was conversely inhibited. **1H-NMR results of strong base component showed that hydrogen aromaticity of feed markedly decreased in H//2 S/H//2 flow but no difference was observed between Ha in H//2 flow and that in H//2S/H//2 flow. Promoting effect of H//2S on HDN reaction was thus ascribed to a improvement of catalytic cracking ability for C-N bond.
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