Oxidative damage to guanine bases initiated by photolysis of the water-soluble radical generator 2,2'-azobis(2-amidinopropane) dihydrochloride (AAPH) has been investigated by laser kinetic spectroscopy. In the neutral oxygenated aqueous solutions, 355 nm laser flash photolysis of AAPH generates a whole spectrum of free radicals including 2-amidinoprop-2-peroxyl (ROO center dot), 2-amidinoprop-2-oxyl (RO center dot), and superoxide (O-2(center dot-)) radicals. These oxyl radicals with negligible absorption in a near UV-visible range were monitored in the reactions leading to the products with characteristic absorption spectra. This approach reveals that RO center dot radicals induce fast, one-electron oxidation of 2'-deoxyguanosine (dG) to form guanine neutral radicals, dG(-H)(center dot). In contrast, ROO center dot radicals do not react at observable rates with dG. The O-2(center dot-) radicals were detected using a classical test reaction with tetranitromethane to form nitroform. The major pathway for formation of the end-products of guanine oxidation is the combination of the G( H)(center dot) and O-2(center dot-) radicals to form 2,5-diamino-4H-imidazolone (Iz). This mechanism was confirmed by analysis of the end-products produced by oxidation of two substrates: (1) the guanosine derivative 2',3',5'-tri-O-acetylguanosine (tri-O-Ac-G) and (2) the 5'-d(CCATCGCTACC) sequence. The major products isolated by HPLC and identified by mass spectrometry methods were the tri-O-Ac-Iz and 5'-d(CCATC[Iz]CTACC products.