The change in the structure and composition of molybdenum nitride catalysts with cooling in a stream of ammonia or helium gas after NH3 treating was determined using temperature-programmed reduction (TPR) and X-ray powder diffraction analyses. The relationship between the molybdenum species and the catalytic activities of the molybdenum nitride catalysts for the hydrodenitrogenation (HDN) of carbazole was discussed. MoO2, gamma-Mo2N, and Mo metal were mainly formed during the temperature-programmed reaction of MoO3 with ammonia at 773, 973, and 1173 K, respectively. During the TPR experiment, a portion of the adsorbed NHx (x = 0-3) species caused further nitriding of the catalyst at higher temperatures. It was found that nitrogen desorption during TPR could be assigned to four types of nitrogen species: (1) NHx adsorbed on MoO2, (2) NHx adsorbed on gamma-Mo2N, (3) N-2 during the transformation of gamma-Mo2N to beta-Mo2N0.78, and (4) N-2 during the reduction of beta-Mo2N0.78 to molybdenum metal. Purging the NH3-treated catalyst with helium at 973 K not only removed the adsorbed NH2 and diffused nitrogen but also altered the structure of the molybdenum compounds, i.e., from gamma-Mo2N to beta-Mo2N0.78 During the HDN of carbazole, gamma-Mo2N was the most active, followed by beta-Mo2N0.78 for CN hydrogenolysis, while molybdenum metal had the highest activity for hydrogenation. NH3-treated MoO2 was much less active for both C-N hydrogenolysis and hydrogenation during carbazole HDN. From these results, the C-N hydrogenolysis sites were most likely located on small nitrogen deficient particles and crystallites of the molybdenum nitrides. The hydrogenation sites were located on the surface grain boundary of molybdenum metals.