The selenite radical, SeO3-, has been found to selectively produce the cytosyl radical upon one-electron oxidation of duplex DNA. This is at first a surprising result as SeO3- can only oxidize guanine of the DNA bases, implying that the transiently formed guanyl radical cation must transpose into the neutral cytosyl radical with loss of a proton. Back oxidation to produce the neutral guanyl radical, in competition with another fixation reaction, is observed.