The kinetics of ammonia decomposition has been studied using a zirconium oxynitride catalyst, which has been synthesized via ammonolysis of zirconia. The catalyst was characterized with BET. XRD and hot gas extraction. The conversion of NH3 was measured as a function of residence time, temperature, partial pressures of H-2 and NH3 in the inlet flow, respectively, via infrared spectrometry. The activation energy of the rate determining step and adsorption enthalpies were determined for a 3 and a 5-step reaction mechanism, considering an interaction between NH3 and the oxygen vacancies of the catalyst. A comparison with theoretically predicted values offered a good agreement and led to a refinement of the mechanism. Research highlights: Reaction mechanism for decomposition of ammonia over zirconium oxynitride. Determining of equilibrium constants and activation energy. Link between theoretical works and experiment. (C) 2010 Elsevier B.V. All rights reserved.