It has been found that the rate of ammonia synthesis over the K-2[Ru-4(CO)(13)]-based catalysts on carbonized magnesium oxide and the original graphite-like active carbon ’Sibunit’ can be considerably enhanced by the introduction of cesium nitrate into the samples followed by reduction of the supported CsNO3 to cesium oxide or hydroxide with dihydrogen. In the case of the catalysts on carbonized magnesium oxide the greatest accelerating effect of cesium is manifested at Cs+:Ru molar ratio of 1.51:1. Both an increase and a decrease in this ratio diminishes the activity of the system in ammonia synthesis. Analogous dependence on the amount of the cesium promoter is observed for the catalysts on ’Sibunit’ carbon but in this case the position of the maximum activity is shifted towards larger cesium contents in the samples. The cesium-promoted catalyst on carbonized MgO is noticeably more active in ammonia synthesis than that on ’Sibunit’ carbon.