Surface reactions between N16O and 18O2 coadsorbed on Pt(111) have been studied by temperature-programmed desorption (TPD), low energy electron diffraction and infrared reflection absorption spectroscopy (IRAS). When the surface covered with N16O and 18O2 is annealed, 18O16O desorbs at 155 K and N18O at 145, 310, and 340 K. In addition, a new absorption line at 1658 cm−1 due to N18O appears above 265 K. However, there is no indication of oxidation products of NO in the TPD and IRAS measurements. Thus, only oxygen-exchange reactions take place on the coadsorbed surface. Reaction yields and mechanisms of the oxygen-exchange strongly depend on the adsorption states of oxygen. At ∼145 K, molecularly adsorbed oxygen and N16O (νNO=∼1930 cm−1) directly interacting with the oxygen molecule are involved in the exchange reaction. Above 270 K the exchange reaction takes place between atop N16O (νNO=∼1720 cm−1) and the metastable oxygen adatoms that do not possess a long-range order. On the contrary, the oxygen-exchange reaction is greatly suppressed when NO is coadsorbed on the surface with well-ordered p(2×2) oxygen atoms.