Oxidation of 2‘-deoxyguanosine dG with the chemical nuclease Mn-TMPyP/KHSO5 leads within one minute to a nearly quantitative amount of 2-amino-5-[(2-deoxy-β-d-erythro-pentofuranosyl)amino]-4H-imidazol-4-one (dIz or imidazolone for short) (90%) in a non-oxygen-dependent pathway and without formation of 8-oxo-dG. This new mechanism of dIz formation involves, as a crucial first step, the abstraction of two electrons from guanine by a high-valent porphyrin−Mn(V)O species, leading to the dG+ cation instead of the classical dG•+ radical cation. We describe also the oxidation of imidazolone into the corresponding imidazolone N-oxide in the presence of KHSO5.