Periodic density functional theory (DFT) calculations using plane waves have been performed to systematically investigate the adsorption and relative stability of ammonia and its dehydrogenated species on Pt(111) and Pt(100) surfaces. Different adsorption geometries and positions have been studied, and in each case, the equilibrium configuration has been determined by relaxation of the system. The vibrational spectra of the various ammonia fragments have been computed, and band assignments have been compared in detail with available experimental data. The adsorption of NH₃ (on top) and NH₂ (bridge) is more favorable on Pt(100) than on Pt(111), while similar adsorption energies were computed for NH (hollow) and N (hollow) on both surfaces. The remarkably lower adsorption energy of NH₂ over Pt(111) as compared with Pt(100) (the difference being ~0.7 eV) can be related to different geometric and electronic factors associated with this particular intermediate. Accordingly, the type of platinum surface determines the most stable NHx fragment: Pt(100) has more affinity for NH² species, whereas NH species are preferred over Pt(111).