Catalytic ammonia oxidn. over platinum has been studied exptl. from UHV up to atm. pressure with polycryst. Pt and with the Pt single crystal orientations (533), (443), (865), and (100). D. functional theory (DFT) calcns. explored the reaction pathways on Pt(111) and Pt(211). It was shown, both in theory and exptl., that ammonia is activated by adsorbed oxygen, i.e. by Oad or by OHad. In situ XPS up to 1 mbar showed the existence of NHx (x = 0,1,2,3) intermediates on Pt(533). Based on a mechanism of ammonia activation via the interaction with Oad/OHad a detailed and a simplified math. model were formulated which reproduced the exptl. data semiquant. From transient expts. in vacuum performed in a transient anal. of products (TAP) reactor it was concluded that N2O is formed by recombination of two NOad species and by a reaction between NOad and NHx,ad (x = 0,1,2) fragments. Reaction-induced morphol. changes were studied with polycryst. Pt in the mbar range and with stepped Pt single crystals as model systems in the range 10-5-10-1 mbar.