In order to better define the mechanism and the products of guanine oxidation within DNA, we investigated the details of the mechanism of guanine oxidation by a metalloporphyrin, Mn‐TMPyP, associated to KHSO5 on oligonucleotides. We found that the three major products of guanine oxidation are formed by independent reaction routes. The oxidized guanidinohydantoin (1) and the proposed spiro compound 3 derivatives are not precursors of imidazolone lesion (Iz). These guanine lesions as well as their degradation products, may account for non‐detected guanine oxidation products on oxidatively damaged DNA.