Periodic DFT calculations using plane waves have been applied to comparatively study the adsorption and decomposition of ammonia on the (111) and (100) surfaces of platinum-group metals (Pd, Rh, Pt). Different adsorption geometries and positions have been studied for NH₃ and its dehydrogenation intermediates (NH_x, x = 0, 1, 2). On the six surfaces investigated, NH₃ adsorbs preferentially on top sites, NH2 on bridge, and NH and N on hollow sites. However, the adsorption energies of the NHx moieties differ considerably from one surface to another. All of the species adsorb more strongly on the (100) than on the (111) planes. Rh(100) provides the maximum stability for the various intermediates. The reaction energies, the structure of the transition states, and the activation barriers of the successive dehydrogenation steps (NH_x —> NH_x-1 + H) have been determined, making it possible to compute rate coefficients at different temperatures. Our calculations have confirmed that ammonia decomposition over noble metal catalysts is structure sensitive. As a general trend, the first dehydrogenation step is rate determining, especially for Pd. In agreement with experiments, Rh is a better catalyst for NH₃ decomposition than are Pt and Pd. The former strongly stabilizes the highly dehydrogenated NH and N species and also leads to the lowest activation barriers. For the set of dehydrogenation reactions, a linear relationship between the transition state potential energy and the adsorption energy of the final state has been obtained.