The aim of the present work was to evaluate the potential for ¹O₂ to induce oxidation of cellular DNA. For this purpose cells were incubated in the presence of a water-soluble endoperoxide whose thermal decomposition leads to the formation of singlet oxygen. Thereafter, DNA was extracted and the level of several modified DNA bases was determined by HPLC analysis coupled to a tandem mass spectrometric detection. A significant increase in the level of 8-oxo-7,8-dihydro-2′-deoxyguanosine was observed upon incubation of the cells with the chemical generator of¹O₂, whereas the level of the other DNA bases measured remained unchanged. To demonstrate that singlet oxygen is directly involved in the formation of 8-oxo-7,8-dihydro-2′-deoxyguanosine, the corresponding¹⁸O-labeled endoperoxide was used. Incubation of the cells with such a generator of ¹⁸O-labeled singlet oxygen results in the formation of ¹⁸O-labeled 8-oxo-7,8-dihydro-2′-deoxyguanosine in the nuclear DNA. This result clearly demonstrates that singlet oxygen, when released within cells, is able to directly oxidize cellular DNA.