The reaction between nitrogen dioxide (NO2 center dot) and guanine radical cation (G(center dot+)) yielding the mutagenic product 8-nitroguanine radical cation (8-nitroG(center dot+)) was studied in the presence of one or two water molecules. All the relevant extrema on the potential energy surface were located by fully optimizing the geometries of the reactant, intermediate, and product complexes as well as transition states at the B3LYP/6-31G**, B3PW91/6-31G**, B3LYP/AUG-cc-pVDZ, and B3PW91/AUG-cc-pVDZ levels of density functional theory in gas phase. Zero point energy-corrected total energies and the corresponding Gibbs free energies at 298.15 K were obtained at the B3LYP/AUG-ce-pVDZ and B3PW91/AUG-cc-pVDZ levels of theory. Single point energy calculations were pet-formed for all the optimized geometries at the MP2/AUG-cc-pVDZ level of theory in gas phase. Solvent effect of aqueous media was treated by performing single point energy calculations at the B3LYP/AUG-cc-pVDZ, B3PW91/AUG-cc-pVDZ, and MP2/AUG-cc-pVDZ levels of theory employing the polarizable continuum model. The solvent effect of bulk water as well as that due to specific water molecules were found to play very important roles in lowering down many barrier energies appreciably. It is found that 8-nitroG(center dot+) complexed with water molecules would be formed due to the reaction of G(center dot+) with NO2 center dot in aqueous media. The possible biological significance of the results obtained has been examined by studying binding energies of several normal and abnormal base pairs.