The reaction of oxide radical ion (. O-) with a variety of pyrimidine and purine derivatives, in strong alkaline aqueous solution (pH 13.7), has been investigated by means of the pulse radiolysis technique. Rate constants for the reaction of . O- species with various substrates are in the range of (2.8-7.8) x 10(8) M-1 s(-1). Absorption spectra of the transients and information on their decay reactions have also been obtained. For comparison, the one-electron oxidation of the same substrates with Br-2(.-) have also been carried out at the same pH when it was necessary. With uracil, cytosine, adenine, and guanine, the oxide radical ion effects a one-electron oxidation, whereas in the presence of extractable hydrogens (e.g., with methyl-substituted derivatives) it abstracts a hydrogen atom from the methyl group. The transient species derived from uracil and adenine decay by first-order kinetics (k(decay) = 1.0 x 10(4) s(-1)), whereas the transient species derived from cytosine and guanine as well as those obtained after hydrogen abstraction from the methyl-substituted derivatives decay by second-order kinetics (2k = (3-9) x 10(8) M-1 s(-1)).