Abstract Exocyclic DNA adducts are considered as potential tools for the study of oxidative stress-related diseases, but an important aspect is their chemical reactivity towards oxidant species. We report here the oxidation of 1-N ²-etheno-2′-deoxyguanosine (1,N ²-εdGuo) by singlet molecular oxygen (¹O₂) generated by a non-ionic water-soluble endoperoxide [N,N′-di(2,3-dihydroxypropyl)-1,4-naphthalenedipropanamide endoperoxide (DHPNO₂)] and its corresponding oxygen isotopically labeled [¹⁸O]-[N,N′-di(2,3-dihydroxypropyl)-1,4- naphthalenedipropanamide endoperoxide (DHPN¹⁸O₂)], and by photosensitization with two different photosensitizers [methylene blue (MB) and Rose Bengal (RB)]. Products detection and characterization were achieved using high performance liquid chromatography (HPLC) coupled to ultraviolet and electrospray ionization (ESI) tandem mass spectrometry, and nuclear magnetic resonance (NMR) analyses. We found that dGuo is regenerated via reaction of ¹O₂ with the ε-linkage, and we propose a dioxetane as an intermediate, which cleaves and loses the aldehyde groups as formate residues, or alternatively, it generates a 1,2-ethanediol adduct. We also report herein the quenching rate constants of ¹O₂ by 1,N ²-εdGuo and other etheno modified nucleosides. The rate constant (k_t ) values obtained for etheno nucleosides are comparable to the k_t of dGuo. From these results, we suggest a possible role of ¹O₂ in the cleanup of etheno adducts by regenerating the normal base.