The main photosensitized oxidation product in aqueous solution of 8-oxo-7,8-dihydro-2’-deoxyguanosine (1) has been isolated by HPLC and characterized as 1-(2-deoxy-beta-D-erythro-pentofuranosyl)-cyanuric acid (7) and its precursor, 1,3,5-triazine-1(2H)-carboximidamide, 3-(2-deoxy-beta-D-erythro-pentofuranosyl)tetrahydro-2,4,6-trioxo-(6). This was achieved by carrying out extensive spectroscopic measurements including FAB mass spectrometry, H-1 and C-13 NMR analyses. The formation of the photooxidized nucleoside 7 is accounted for by a type II mechanism through initial [2+2] cycloaddition across the C-4-C-5 ethylenic bond of 1. The 1,2 bond cleavage of the transient dioxetanes gives rise to the modified nucleoside 6 through a nine-membered ring oxidized intermediate nucleoside. The derivative 6 further decomposes into the stable cyanuric acid nucleoside 7 by the hydrolysis of the guanidino residue with the concomitant release of urea 8. Photosensitizers such as methylene blue and rose Bengal were found to efficiently photooxidize 8-oxodGuo 1 via a type TI mechanism (O-1(2)), whereas riboflavin and benzophenone, which act mostly by a type I mechanism, were far less efficient.