Pulse radiolysis measurements of the decay of hydrated.. electrons in solutions containing different concentrations of the oligonucleotide GTG with and without a cisplatin adduct show that the presence of a cisplatin moiety accelerates the reaction between hydrated: electrons and the oligonucleotide: The rate constant of the reaction is foond to be 2.23 x 10(10) mol(-1) L s(-1), which indicates that it is diffusion. controlled. In addition, we show for the first time the formation of a Pt-I intermediate as a result of the reaction of hydrated elections With GTG-cisplatin. A putative reaction mechanism is proposed, Which limy form the basis of the radiosensititation of cancer cells in concomitant chemoratiation therapy with cisplatin.