High surface area (>170 m² g⁻¹) molybdenum nitride was prepared by the temperature-programmed nitridation of α-MoO₃ with pure ammonia. The process was optimized by adjusting the experimental variables: the reaction temperature, heating rate, and molar flow rate of ammonia. The physicochemical properties of the as-formed molybdenum nitride were characterized by X-ray diffraction, N₂ sorption, transmission electron microscopy, temperature-programmed oxidation/reduction, and X-ray photoelectron spectroscopy. Of the experimental variables, the nitridation temperature was found to be the most critical parameter determining the surface area of the molybdenum nitride. When the prepared molybdenum nitride was exposed to air, the specific surface area rapidly decreased because of the partial oxidation of molybdenum nitride to molybdenum oxynitride. However, the surface area recovered to 90% the initial value after H₂ treatment. The catalyst with the highest degree of nitridation showed the best catalytic activity, superior to that of unmodified α-MoO₃, for the decomposition of ammonia because of its high surface area.