A study of the surface chemistry of coadsorbed NO and O 2 on Pt(111) using electron energy loss spectroscopy (EELS), temperature programmed desorption (TPD), and photon induced desorption spectroscopy (PID) is presented. A 1 4 N 1 8 O isotope exchange reaction product is detected in TPD of 1 4 N 1 6 O/ 1 8 O 2 /Pt(111) with the primary desorption peak occurring at 315 K. The wavelength dependence of photodesorption of 1 4 N 1 8 O, 1 4 N 1 6 O, and O 2 from 1 4 N 1 6 O/ 1 8 O 2 /Pt(111) at 85 K with photons between 240 and 950 nm was measured, with that of 1 4 N 1 8 O found to be very similar to O 2 photodesorption. EELS features at 95 meV (δ(ONO)] and 155 meV (ν s (ONO)] reveal the production of NO 2 upon NO adsorption on O 2 /Pt(111) at 90 K. A new adsorption state of NO, denoted γNO, formed upon coadsorption of NO with O 2 on Pt(111) was characterized by a unique vibrational signature in EELS [ν γ (N O) = 236 meV]. γNO was found to have a photodesorption cross section approximately 4000 times greater than that of NO adsorbed alone on clean Pt(111) due to photoyield enhancement by coadsorbed O 2 , while exhibiting a similar wavelength dependence to NO/Pt(111).