Biphotonic photoionization of frozen aqueous solutions of DNA at 248 nm has been shown by EPR spectroscopy to lead selectively to the guanine cation. In (H2O)-O-18 under these conditions high levels of [O-18]-7,8-dihydro-8-oxo-2’-deoxyguanosine are produced in a dose-dependent manner, confirming direct formation of this oxidation product by hydration of the guanine cation. Photoionization of defined oligonucleotides did not give rise to significant levels of immediate strand breaks but generated G-specific alkali-labile sites that are readily cleaved by piperidine treatment, Authentic oligonucleotides containing 7,8-dihydro-8-oxo-2’-deoxyguanosine (8-oxodG, 5) sites are slowly cleaved at these sites on treatment with piperidine but at rates inconsistent with this being the source of the alkali-labile site, Photoionization of 7,8-dihydro-8-oxo-2’-deoxyguanosine-cont oligonucleotides demonstrated that this residue is highly susceptible to secondary oxidation and leads to formation of a markedly more alkali-labile lesion. Photoionization of 7,8-dihydro-8-oxo-2’-deoxyguanosine leads to release of 7,8-dihydro-8-oxoguanine suggesting that the alkali-labile site on photoionization of 7,8-dihydro-8-oxo-2’-deoxyguanosine-containing oligonucleotides is an apurinic site. However, the lack of significant 7,8-dihydro-8-oxoguanine release on photoionization of the parent oligonucleotides rules out secondary oxidation of 7,8-dihydro-8-oxo-2’-deoxyguanosine as the mechanism of formation of alkali-labile sites.