The one-electron oxidation of guanine in the GC base pair of DNA has been investigated using pulse radiolysis combined with DFT calculations. Reaction of benzotriazinyl radicals with DNA results in the formation of the neutral guanyl radical and redox equilibria. The one-electron reduction potential, E(7), of the neutral guanyl radical in the GC base pair is determined for the first time as 1.22 +/- 0.02 V, from both absorption and kinetic data.