Chemical radiolytic methods were used to investigate the reactions of hydrated electrons (e(aq)(-)) with 8-bromoguanosine (8-Br-Guo) as a function of pH. gamma-Radiolysis of 8-Br-Guo in aqueous solutions followed by product studies showed the formation of guanosine (Guo) as a single product at various pH. In D2O solutions the quantitative incorporation of deuterium at the 8-position was also observed. Pulse radiolysis revealed the "instantaneous" formation of a guanosine radical cation (Guo(.+) or its deprotonated forms) in acid or basic solutions. The same transient species results from the reaction of H-. with 8-Br-Guo at pH 3, as well as from the reaction of(CH3)(2)CO.- with 8-Br-Guo at pH 13. In neutral solution, the initial electron adduct was rapidly protonated to give the first observable transient species that decays by first-order kinetics (k = 5 x 10(4) s(-1)) to produce the Guo(-H+)(.) radical once again. Tailored experiments allowed the reaction mechanism to be defined in some detail.