Positron emission profiling (PEP) is applied to study the adsorption and dissociation of ammonia on metallic and preoxidized platinum sponge in the temperature range 323–573 K. The results show that ammonia weakly interacts with platinum without dissociation. On the contrary, preoxidized platinum is able to dissociate ammonia. With increasing temperature more ammonia is converted into N2, N2O, and H2O. Adsorbed NO appears to be an important intermediate, while its formation strongly depends on the oxygen surface coverage. Temperature-programmed experiments are performed to characterize the adsorbed nitrogen species. Furthermore, H2, NH3, and NO are used to react with these adsorbed nitrogen species. These experiments indicate that mainly NHx species are present at the platinum surface.