The reduction of molybdenum dioxide in the temperature range 450–550°C leads to a finely divided molybdenum powder which can readily be converted to molybdenum nitride (γ-Mo2N) or used as a catalyst for the synthesis of ammonia at temperatures between 300 and 500°C. Little chemisorption of nitrogen occurs on pure molybdenum dioxide which also shows negligible activity for the synthesis of ammonia except under conditions such that reduction can also occur. A constant specific activity for the formation of ammonia is found on all catalysts after exposure to the synthesis gas for a few hours at each reaction temperature. This specific activity is independent of the nature of the bulk of the solid which may be oxide, metal or nitride or a combination of all three phases. The activation energy for the synthesis is 14 kcal/mole and ancillary experiments show that the rate of the process is controlled by one of the steps in the conversion of adsorbed nitrogen to ammonia and not by the rate of chemisorption as suggested by Kiperman and Temkin.2 The hydrogenation of molybdenum nitride takes place more slowly than the synthesis on the same sample of catalyst but the two processes are closely related. New data are reported on the free energy of formation of the nitride in the temperature range 425–550°C.