The formation and stability of an ammonia synthesis catalyst, based on cobalt molybdenum nitrides, were studied. The activation process of the catalyst was examined by in situ X-ray diffractometry. The thermal stability of obtained active phase of the catalyst was tested at 700 °C under ammonia atmosphere, N 2 /H 2 mixture and under pure hydrogen. The presence of Co 2 Mo 3 N and Co 3 Mo 3 N phases in the catalyst was confirmed. The phase composition was stable in a long-term test performed under nitrogen/hydrogen atmosphere. Co 3 Mo 3 N phase decomposed into Co 6 Mo 6 N after exposure to pure hydrogen.