The relationship between the surface molybdenum species and adsorbed nitrogen species on nitrided 1.0-18.7% MoO3/Al2O3 was elucidated by XPS and temperature-programmed reduction (TPR). The MoO3/Al2O3 samples were nitrided by temperature-programmed reaction with NH3. From the XPS analysis, Mo3+ and Mo4+ ions were predominant on the surface of the nitrided Mo/Al2O3 samples. From the TPR measurement, the ammonia desorption was due to nitrogen species adsorbed on alumina. The nitrogen desorption was due to two kinds of nitrogen desorption from the structures of gamma-Mo2N and beta-Mo2N0.78 and four kinds of nitrogen desorption from NHX species adsorbed on MoO2, Mo3+ ion (gamma-Mo2N), Mo2+ ion (molybdenum nitride on alumina such as highly dispersed molybdenum nitride), and alumina.