Promoted nanocrystalline iron was nitrided in a differential reactor equipped with systems that made it possible to conduct both thermogravimetric measurements and hydrogen concentration analyses in the reacting gas mixture. The nitriding process, particularly catalytic ammonia decomposition reaction, was investigated under an atmosphere of ammonia-hydrogen mixtures, under atmospheric pressure. Ammonia concentrations, and so nitriding potentials, were changed gradually from 0 to 100% at the inlet of reactor. The temperature was changed in the range of 475-500 degrees C. While values of nitriding potential were increasing, the rate of catalytic ammonia decomposition on alpha-Fe(N) was increasing too, but on mixture of alpha-Fe(N) with gamma'-Fe(4)N nitride the rate was decreasing. The obtained results were interpreted on the basis of the adsorption range model. New equations describing the catalytic ammonia decomposition reaction rate as a function of the logarithm of the nitriding potential of the gas phase, temperature, and nitriding degree of solid samples were proposed.