Calf thymus DNA and 8-oxo-7,8-dihydro-2’-deoxyguanosine (8-oxodGuo) were photooxidized in the dark by triplet-excited ketones generated in the thermal decomposition of 3-(hydroxymethyl)-3,4,4-trimethyl-1,2-dioxetane (HTMD). The oxidation of DNA led to 8-oxodGuo and the type I photooxidation product 2,2-diamino[2-deoxy-beta-D-erythro-pentofuranosyl)-4-amino]-5(2H)-oxazolone (oxazolone). While the yield of oxazolone progressively increased, 8-oxodGuo was substantially consumed in DNA on successive exposure to HTMD. The oxidation of authentic 8-oxodGuo by HTMD and established photosensitizers such as benzophenone (mainly type I) and Rose Bengal (predominantly type II) was studied in detail in regard to the concentration and time dependence and the influence of D2O versus H2O. The singlet-oxygen-derived 4R* and 4S* diastereomers of 4-hydroxy-8-oxo-4,8-dihydro-2’-deoxyguanosine (4-HO-8-oxodGuo) and oxazolone were the major products. A substantial kinetic D2O effect (ca. 10-fold) in the Rose Bengal-photosensitized degradation of 8-oxodGuo unequivocally established that in this case singlet oxygen (type II photooxidation) is involved. However, the efficient formation of oxazolone by benzophenone as a characteristic type I photooxidant, as well as in the HTMD-mediated oxidation (predominantly type I), and the fact that these processes exhibit a negligible D2O effect provide cogent experimental evidence for an electron or hydrogen atom transfer mechanism (type I photooxidation) in the oxidative degradation of 8-oxodGuo into oxazolone. The unprecedented observation that comparable product ratios of 4-HO-8-oxodGuo and oxazolone were obtained in the 8-oxodGuo oxidations, irrespective of whether Rose Bengal as a typical type II photooxidant or benzophenone as an established type I photooxidant was employed, is presumably due to electron-transfer chemistry of O-1(2) With the easily oxidized 8-oxodGuo in view of its low oxidation potential. This nicely accounts for the fact that the primary oxidation product 8-oxodGuo, which serves as important monitor of-oxidative genotoxicity, may not accumulate appreciably in the photooxidation of DNA.