The NO oxidation reaction on platinum is the first step in the NOx storage and reduction process (NSR), a technology currently being commercialized for emissions reduction in diesel engines. The kinetics on Pt(111) and Pt(110) single crystals at the same conditions used on supported catalysts were determined. The surface coverage of oxygen and nitrogen species as well as the oxidation state of Pt under simulated reaction conditions was studied by in-situ XPS. In a series of experiments with NO and NO2, the ratio of NO/NO2 played a critical role in determining the coverage of oxygen on the surface and the availability of empty sites for adsorption of O2, the proposed rate determining step in our L-H mechanism. The experiments showed why the product NO2 effectively inhibits the forward reaction rate by poisoning the surface with oxygen. Based on the kinetics and in-situ XPS results, large platinum particles resisted oxide formation due to the closed-packed structure and remained active whereas small particles which had a more open structure on average were more easily oxidized and became inactive. This is an abstract of a paper presented at the 2007 AIChE Annual Meeting (Salt Lake City, UT 11/4-9/2007).