Ru–Ba–Cs/Sibunit ternary catalysts were synthesized. Their activity in ammonia synthesis and the thermal stability of the support in a reducing atmosphere at elevated temperature depend on the molar ratio of the promoters. The results of studies using physical methods suggest that cesium predominantly interacts with the support, acting as an electronic promoter, whereas barium is a structural promoter. The synergistic action of the promoters at increased barium content of the catalyst was revealed. The highest activity in ammonia synthesis, 34.5 mLNH3 gcat –1 h–1, was reached for the 4% Ru–10.8% Ba–2.6% Cs/Sibunit catalyst.