A series of ruthenium NH3 synthesis catalysts with selected promoters added have been studied. It is demonstrated that the rate of N-2 chemisorption can be strongly enhanced by preadsorbed hydrogen. The promoters La and Ba have a weak influence on the rate of N-2 dissociation on the hydrogen free surface, while they cause the rate to increase for a partly hydrogen covered surface. This increase correlates with the effect on the rate of NH3 synthesis. In contrast, Cs was able to increase the rate of N-2 dissociation by two to three orders of magnitude with almost no change in the activity for NH3 synthesis.