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Cobalt molybdenum nitrides are one of the most promising ammonia-synthesis catalysts of the future. However, the selection of the optimal promoter composition is a challenging task. In this paper, the structure–property relationship of ternary cobalt molybdenum nitrides as ammonia synthesis catalysts promoted with potassium was studied. A series of catalysts containing 0.2–3.5 wt% potassium was analyzed by means of X-ray diffraction, volumetric gas adsorption, and activity tests in ammonia synthesis. The catalysts were subjected to thermal aging in the same catalytical reactor. The influence of the potassium promoter on the thermostability was determined. The observed loss of activity in catalysts with a high potassium content was related to the concentration of Co₂Mo₃N and Co₃Mo₃N phases, the mean crystallite sizes, the specific surface area, and the pore size distribution.