A picosecond flash-photolysis study of the ionization of psoralen derivatives and ethidium bromide, either free or intercalated in DNA, was carried out. A single short high-intensity 355 nm laser pulse with a few tens of J/cm2 dose, was found to be sufficient for achieving close to the ultimate 100% yield of ionization of either free or DNAbound psoralens. Interestingly, the photoionization efficiency of ethidium bromide is considerably reduced upon intercalation.