Steady-state isotopic transient kinetic analysis was used to examine a cesium-promoted Ru/MgO catalyst (1.75 wt% Ru) for ammonia synthesis. For comparison, an unpromoted Ru/SiO2 (22 wt% Ru) catalyst was also studied. Steady-state and isotopic transient measurements were conducted at temperatures between 603 and 673 K with a stoichiometric ratio of N-2 and H-2 at a total pressure of 3 atm. The fractional surface coverage of nitrogen-containing intermediates (theta(NHx)) based on total H-2 chemisorption was found to range from 0.02 to 0.05 on both catalysts under the conditions used in the current study. However, the intrinsic turnover frequency of ammonia synthesis over Cs-Ru/MgO was two orders of magnitude greater than that of the unpromoted Ru/SiO2. The combination of the Cs promoter and MgO support enhanced the intrinsic activity of Ru, presumably by lowering the barrier for dinitrogen dissociation.