The dissociative chemisorption of N-2 is generally accepted to be the rate-determining step of ammonia synthesis over Ru-based catalysts. The interaction of N-2 with the following three Ru catalysts has been studied : Ru supported on Al2O3 (Ru/Al2O3) and on MgO (Ru/MgO), and Ru/MgO promoted with cesium (Cs-Ru/MgO). Temperature-programmed N-2 adsorption and desorption experiments and the isotopic exchange reaction (NN)-N-14-N-14 + (NN)-N-15-N-15 reversible arrow 2(14)N(15)N were performed in a microreactor flow system. A microkinetic analysis based on the Langmuir-Hinshelwood Hougen-Watson mechanism has been applied to these kinetic experiments yielding the rate constants of dissociative chemisorption (k(ads)) and associative desorption (k(des)). The dissociation of N-2 was indeed found to be a slow and activated process. Ru/Al2O3 was found to be rather inactive for N-2 dissociation. Ru/MgO turned out to be a heterogeneous system with respect to the interaction with N-2 due to the presence of promoted active sites which dominate the rate of N-2 dissociation. Promotion by cesium was observed to enhance both k(ads) and k(des) significantly and rendered the Ru metal surfaces uniform toward the interaction with N-2. The initial sticking coefficient and the rate of desorption of N-2 derived from the microkinetic models are in good agreement with results obtained with Ru single crystal surfaces under ultra-high vacuum conditions.