The catalytic oxidation of NH3 over a polycrystalline Pt surface in the temperature range 800–1400 K has been investigated with special emphasis on radical production through the use of laser-induced fluorescence detection. Among all the free radicals and transient species probed (NH, NH2, OH, HNO, and NO2) only the diatomic species NH and OH were detected. NH production was diminished drastically by O2 and yielded a desorption energy of 66 kcal/mol. The activation energy for OH desorption was found to depend strongly on the O2/NH3 ratio. Its desorption energy varied from 26 kcal/mol at O2/NH3 = 100 to a peak value of 42 kcal/mol near the stoichiometric ratios for N2 and NO production (O2/NH3 = 1) and then down to the value of 26 kcal/mol at O2/NH3 = 0.03. This interesting variation in OH desorption energy is attributed to site competition between O and N atoms previously noted for NO and N2 production. An atomic/radical mechanism of NH3 oxidation on Pt was proposed to account for the formation of these radical products.