Ammonia oxidation with oxygen over Pt(533) in the pressure range 10(-7)-10(-3) mbar has been studied using LEED and scanning tunneling microscopy (STM) to detect reaction-induced structural changes. Below 620 K, strong hysteresis effects in the rate occur upon cyclic variation of one of the p, T parameters. The rate changes are associated with structural modi cations of the substrate. The Pt(533) surface exhibiting single atomic steps after preparation undergoes a doubling in step height and terrace width connected with a shift in selectivity from preferential N-2 production to strong NO formation. Restoring the single atomic steps in vacuum requires heating above 1100 K, but under reaction conditions the substrate changes are reversible at least down to 600 K.