Constant CO(x)-free H(2) production from the catalytic decomposition of ammonia could be achieved over a high surface area molybdenum carbide catalyst prepared by a temperature programmed reduction-carburization method. The fresh and used catalyst was characterized by N(2)-adsorption/desorption, powder X-ray diffraction, scanning and transmission electron microscopes and electron energy-loss spectroscopy at different stages. Observed deactivation (in the first 15 hours) of the high surface area carbide during reaction was ascribed to considerable reduction of the specific surface area due to nitridation of the carbide under reaction condition. Theoretical calculations confirm that the N atoms tend to occupy subsurface sites, leading to the formation of nitride under NH(3) atmosphere. The relatively high rate of reaction (30 mmol/g(cat).min) observed for the catalytic decomposition of NH(3) is ascribed to highly energetic sites (twin-boundaries, stacking faults, steps and defects) which are observed in both the molybdenum carbide and nitride samples. The prevalence of such sites in the as-synthesised material results in a much higher H(2) production rate in comparison with previously reported Mo-based catalysts.