The nucleoside 2‘-deoxyguanosine (dGuo) was treated with 3-(hydroxymethyl)-3,4,4-trimethyl-1,2-dioxetane (HTMD), the latter generates efficiently triplet-excited carbonyl products on thermal decomposition in the dark. The type I photooxidation products, 2,2-diamino-[(2-deoxy-β-d-erythro-pentofuranosyl)-4-amino]-5(2H)-oxazolone (oxazolone) and the cyclic nucleoside 2-(S)-2,5‘-anhydro-1-(2-deoxy-β-d-erythro-pentofuranosyl)-5-guanidinylidene-2-hydroxy-4-oxoimidazolidine (oxoimidazolidine), as well as the type II photooxidation products 4-(R)- and 4-(S)-4-hydroxy-8-oxo-4,8-dihydro-2‘-deoxyguanosine (4-HO-8-oxodGuo) and 8-oxo-7,8-dihydro-2‘-deoxyguanosine (8-oxodGuo), were quantitatively determined by appropriate selective and sensitive HPLC assays. The concentration and time profiles revealed that about 40% of the triplet ketones derived from the thermal decomposition of HTMD led to photooxidation of dGuo. Essentially equal amounts of type I and type II photooxidation products were found, as could be established by comparison with predominant type I (benzophenone, riboflavin) and type II (Rose Bengal, methylene blue) photosensitizers. The participation of singlet oxygen (type II activity) was confirmed by the substantial D2O effect in the formation of 8-oxodGuo. The results demonstrate that dioxetanes, particularly HTMD, are efficient photooxidants of dGuo on thermal activation in the dark and constitute excellent chemical tools to study photobiological processes without the use of light, in the present case, photogenotoxicity.