This study targets the kinetics and product detection of the gas-phase oxidation reaction of the protonated 5-dehydrouracil (uracil-5-yl) distonic radical cation using ion-trap mass spectrometry. Protonated 5-dehydrouracil radical ions (5-dehydrouracilH+ radical ion, m/z 112) are produced within an ion trap by laser photolysis of protonated 5-iodouracil. Storage of the 5-dehydrouracilH + radical ion in the presence of controlled concentration of O 2 reveals two main products. The major reaction product pathway is assigned as the formation of protonated 2-hydroxypyrimidine-4,5-dione (m/z 127) + •OH. A second product ion (m/z 99), putatively assigned as a five-member-ring ketone structure, is tentatively explained as arising from the decarbonyl ation (-CO) of protonated 2-hydroxypyrimidine-4,5-dione. Because protonation of the 5-dehydrouracil radical likely forms a dienol structure, the O2 reaction at the 5 position is ortho to an -OH group. Following this addition of O2 , the peroxyl-radical intermediate isomerizes by H atom transfer from the -OH group. The ensuing hydroperoxide then decomposes to eliminate •OH radical. It is shown that this elimination of •OH radical (-17 Da) is evidence for the presence of an -OH group ortho to the initial phenyl radical site, in good accord with calculations. The subsequent CO loss mechanism, to form the aforementioned five-member-ring structure, is unclear, but some pathways are discussed. By following the kinetics of the reaction, the room temperature second-order rate coefficient of the 5-dehydrouracilH + distonic radical cation with molecular oxygen is measured at 7.2 × 10 -11 cm3 molecule-1 s-1 , φ = 12% (with ±50% total accuracy). For aryl radical reactions with O2 , the presence of the •OH elimination product pathway, following the peroxyl-radical formation, is an indicator of an -OH group ortho to the radical site.