Microkinetic models have been established successfully in case of ammonia synthesis over iron (Dumesic and Trevino, 1989) and methanol synthesis over copper (Rasmussen et al., 1994) based on a detailed knowledge about the kinetics of the involved elementary steps which allow to extrapolate the results from single crystal studies to predict the performance of an industrial catalyst operating at high pressure. In the present study, it has been possible to set up a consistent microkinetic model for ammonia synthesis based on a LHHW mechanism which describes the results of steady-state and transient experiments performed with a Cs-promoted ruthenium catalyst supported on MgO in agreement with kinetic data obtained under ultrahigh vacuum conditions.