Short oligonucleotides TTTTTGTGTTT and TTTTTTTGTTT in solution with and without cisplatin (cisPt) bound to the guanine bases were irradiated with gamma-rays at doses varying from 0 to 2500 Gy. To determine the effect of hydrated electrons from water radiolysis on the oligonudeotides, we quenched (OH)-O-center dot radicals with ethylenediaminetetraacetic acid (EDTA) and displaced oxygen, which reacts with hydrated electrons, by bubbling the solution with wet nitrogen. DNA strand breaks and platinum detachment were quantified by gel electrophoresis. Our results demonstrate that hydrated electrons react almost exclusively at the position of the cisPt adduct, where they induce cisPt detachment from one or both guanines in the oligonudeotide. Given the high yield of hydrated electrons in irradiated tissues, this reaction may be an important step in the mechanism of radiosensitization of DNA by cisPt