Mo2C/Al2O3 and Mo2N/Al2O3 catalysts have been synthesized and characterized by X-ray diffraction, pulsed chemisorption (CO and O-2) and infrared (IR) spectroscopy, and temperature programmed desorption (TPD) measurements. Thiophene hydrodesulfurization (HDS) activities were measured for alumina-supported Mo carbide and nitride catalysts as well as a sulfided Mo/Al2O3 catalyst, all with a 10 wt% Mo loading. The HDS activities (mu mol Th/mol Mo/s) of the catalysts after 24 h at 693 K were found to increase according to the trend sulfided Mo < gamma-Mo2N < beta-Mo2C, and provide further evidence that carbide and nitride catalysts have the potential to replace sulfided Mo catalysts in commercial HDS reactors. X-ray diffraction analysis of fresh and tested 30 wt% Mo2C/Al2O3 and Mo2N/Al2O3 catalysts indicates that the bulk structure of the carbide (beta-Mo2C) and nitride (gamma-Mo2N) particles is retained, while IR spectroscopy of adsorbed CO indicates that the surface of the carbide and nitride particles becomes sulfided under reaction conditions. A model is proposed for the structure of the active catalytic surface of the alumina-supported Mo carbide and nitride catalysts in which a thin layer of highly dispersed sulfided Mo is present on the surfaces of the carbide and nitride particles.