Alumina-supported beta-Mo2C, alpha-MoC(1-x) (x approximate to 0.5) and gamma-Mo2N catalysts with Mo loadings in the range 1.5-20 wt% have been synthesized and characterized by X-ray diffraction and pulsed chemisorption (CO and O-2) measurements. Thiophene hydrodesulfurization (HDS) activities were measured for the alumina-supported Mo carbide and nitride catalysts as well as sulfided Mo/Al2O3 catalysts with the same Mo loadings. The HDS activities of the catalysts after 24 h at 693 K were observed to increase according to the trend : sulfided Mo approximate to alpha-MoC(1-x) < gamma-Mo2N < beta-Mo2C. The HDS activities of the different catalysts were found to correlate in a linear fashion with their CO and O-2 chemisorption capacities for the wide range of Mo loadings investigated. Thiophene HDS turnover frequencies for the Mo carbide, nitride, and sulfided Mo catalysts are nearly identical, providing further evidence that the higher HDS activity of alumina-supported beta-Mo2C and gamma-Mo2N catalysts stems from a higher density of active sites.