A platinum(IV) complex with a high reduction potential, d,l-1,2-diaminocyclohexyltetrachloroplatinum(IV) (Pt-IV(dach)Cl-4), oxidizes guanine in guanosine-5'-monophosphate (GMP), 2'-deoxyguanosine-5'-monophosphate (dGMP), d(GG), and a double-stranded oligonucleotide to 8-oxoguanine. To the best of our knowledge, this is the first report that provides unambiguous evidence of DNA oxidation by a Pt-IV complex. This oxidative damage may differentiate the anticancer activity of Pt-IV complexes from their Pt-II analogues.