Addition reaction of all OH radical at the C2, C4, C5 or C8 position of 8-oxoguanine (8OG) and abstraction of its H9 atom by all OH radical were studied using density functional theory (B3LYP) employing 6-31G**, 6-311++G** and AUG-cc-pVDZ basis sets. Solvent effects of aqueous media were treated using the PCM model. It is found that the addition of an OH radical at the C4 position of 8OG would be most favored in both gas phase and aqueous media. These addition and abstraction reactions in aqueous media are both found to be barrierless. (c) 2006 Elsevier B.V. All rights reserved.