The adsorption and dissociation of NHx fragments on the Rh(111) crystal surface have been studied using first-principles density-functional calculations. The stability and configurations of NHx species have been investigated and characterized using frequency analysis. The highest adsorption energies have been calculated for NH and N. Several paths of NHx (x=1–3) dehydrogenation and hydrogenation have been taken into account. The transition states have been determined and in detail analyzed. The activation barriers and thermodynamic and kinetic data have been calculated for all the elementary steps. The calculations have shown that the elementary reactions have significant barriers, between 0.7 and 1.1 eV. The transition states are regarded neither early nor late with respect to the distance in hyperspace between initial and final states. The NH3 dehydrogenation has been determined as the rate limiting step. For this elementary process there has been estimated a large contribution of the zero point energy to the activation barrier and a significant entropy activation.