The effects of Cs or Ba promoters on the global and intrinsic turnover frequency of ammonia synthesis over ruthenium supported on carbon (9.1 wt% Ru) were examined at 3 atm and stoichiometric conditions by steady state and isotopic transient measurements at 573 K. Although the global turnover frequency based on total surface Ru atoms was nearly independent of promoter, Cs was more effective than Ba at promoting the intrinsic turnover frequency determined by the isotopic switching experiment. The similarity of the global turnover frequency over the two samples resulted from the coverage of nitrogen-containing intermediates being greater on the Ba-promoted catalyst. The promoters presumably affect the global activity of ammonia synthesis by modifying both the inhibition by dihydrogen and the activation barrier for dinitrogen dissociation.