The study of the biological consequences of oxidative damage to DNA requires the characterization of DNA lesions at a molecular level. The oxidation of guanine in the dinucleoside monophosphate, d(GpT), by a metal-oxo porphyrin produces a dehydroguanidinohydantoin derivative. The(1)H NMR spectrum of this oxidized guanine derivative is reported. The structure was confirmed by the chemical transformation of the dehydroguanidinohydantoin derivative into a linear oxaluric acid derivative upon hydrolysis at ambient temperature. The proposed mechanism of formation of the dehydro-guanidinohydantoin derivative is in agreement with the labeling experiments performed with (H2O)-O-18.