Mechanisms of formation of the mutagenic product 8-oxoguanine (8OG) due to reactions of guanine with two separate OH^• radicals and with H₂O₂ were investigated at the B3LYP/6-31G**, B3LYP/6-311++G**, and B3LYP/AUG-cc-pVDZ levels of theory. Single point energy calculations were carried out with the MP2/AUG-cc-pVDZ method employing the optimized geometries at the B3LYP/AUG-cc-pVDZ level. Solvent effect was treated using the PCM and IEF-PCM models. Reactions of two separate OH^• radicals and H₂O₂ with the C2 position of 5-methylimidazole (5MI) were investigated taking 5MI as a model to study reactions at the C8 position of guanine. The addition reaction of an OH^• radical at the C8 position of guanine is found to be nearly barrierless while the corresponding adduct is quite stable. The reaction of a second OH^• radical at the C8 position of guanine leading to the formation of 8OG complexed with a water molecule can take place according to two different mechanisms, involving two steps each. According to one mechanism, at the first step, 8-hydroxyguanine (8OHG) complexed with a water molecule is formed ,while at the second step, 8OHG is tautomerized to 8OG. In the other mechanism, at the first step, an intermediate complexed (IC) with a water molecule is formed, the five-membered ring of which is open, while at the second step, the five-membered ring is closed and a hydrogen bonded complex of 8OG with a water molecule is formed. The reaction of H₂O₂ with guanine leading to the formation of 8OG complexed with a water molecule can also take place in accordance with two different mechanisms having two steps each. At the first step of one mechanism, H₂O₂ is dissociated into two OH^• groups that react with guanine to form the same IC as that formed in the reaction with two separate OH^• radicals, and the subsequent step of this mechanism is also the same as that of the reaction of guanine with two separate OH^• radicals. At the first step of the other mechanism of the reaction of guanine with H₂O₂, the latter molecule is dissociated into a hydrogen atom and an OOH^• group which become bonded to the N7 and C8 atoms of guanine, respectively. At the second step of this mechanism, the OOH^• group is dissociated into an oxygen atom and an OH^• group, the former becomes bonded to the C8 atom of guanine while the latter abstracts the H8 atom bonded to C8, thus producing 8OG complexed with a water molecule. Solvent effects of the aqueous medium on certain reaction barriers and released energies are appreciable. 5MI works as a satisfactory model for a qualitative study of the reactions of two separate OH^• radicals or H₂O₂ occurring at the C8 position of guanine.