Ammonium-substituted anthraquinone derivatives have been found to catalyze DNA cleavage upon irradiation with 350 nm light. Picosecond laser spectroscopy demonstrates that the excited quinones react with DNA by two separate oxidative pathways : electron transfer from a nearby base and hydrogen atom abstraction from the deoxyribose component of the nucleic acid backbone. Photo-oxidation of DNA by either pathway yields the one-electron-reduced form of the quinone as a byproduct. Back electron transfer from the reduced quinone to the oxidized base is the predominant decay path for the reduced quinone formed by the electron transfer pathway while that formed by hydrogen atom abstraction is converted back to the ground-state, fully oxidized form by reduction of molecular oxygen. The resulting superoxide anion is detected by its reduction of cytochrome c. Comparison of the spectroscopic data for two quinones with significantly different cleavage efficiencies suggests that DNA cleavage by these quinone reagents results from the hydrogen-abstraction chemistry. The laser experiments further indicate that the quinone completes one cycle of excitation, oxidation of DNA, and reduction of oxygen within 10 mu s, illustrating the potential utility of these agents for double-stranded cleavage of DNA.