The reduction of cobalt molybdenum oxide under an ammonia atmosphere resulting in the formation of ternary interstitial nitride Co3Mo3N was studied. Intermediate phases were identified by an in situ powder X-ray diffraction using a reaction chamber. It was supplemented by a thermogravimetric analysis of the process. The presence of intermediate phases, CoMoO4, Co2Mo3O8, Mo2N, metallic cobalt, and Co2Mo3N, was observed. A synthesis route of Co3Mo3N by an ammonolysis method was proposed.