A new method for in situ surface characterization of preferentially oriented platinum nanoparticles is presented. The method is based on the use of structure-sensitive surface reactions such as the redox behavior of underpotential and spontaneous deposition of adatoms. Examples of its use for surface characterization of Pt nanoparticles synthesized by different methods are given. The method has been also employed to rationalize the different behavior of these Pt nanoparticles for the ammonia electro-oxidation in a basic medium.