The deactivation and sulfidation processes of nitrided 12.5% Mo/Al2O3 catalysts at the initial stage were studied on the basis of the behavior of sulfur and nitrogen using XPS spectroscopy. The hydrodesulfurization (HDS) of dibenzothiophene was carried out in a fixed-bed microreactor at 573 K and 10.1 MPa of total pressure. The Mo/Al2O3 catalyst was nitrided by a temperature-programmed reaction with pure ammonia at 4 L h(-1) at various temperatures. From XPS measurement, the sulfur atoms removed from dibenzothiophene were not exchanged with nitrogen atoms in the nitride catalyst during the first hour but molybdenum was sulfided by 71% of the total sulfur accumulated during the 14-h run. The sulfur deposition followed the Elovich equation. The decreased HDS activity and increased hydrogenation selectivity of the nitride Mo/Al2O3 catalyst were caused by the accumulation of sulfur on the nitride catalyst. At steady state achieved after 14 h, however, the nitrogen achieved after the Mo/Al2O3 catalysts were nitrided at high temperatures was difficult to exchange by deposition of sulfur and the release of nitrogen from the nitride catalyst was hampered. The regeneration of the aged nitride catalysts with NH3 after the 14 h-run increased the activity and decreased the hydrogenation selectivity. The mechanism of the regeneration of the aged nitride catalyst by NH3 retreatment and the exchange of oxygen or nitrogen atoms with sulfur atoms in the HDS of dibenzothiophene on the nitrided Mo/Al2O3 catalyst are also discussed.