The carbon dioxide radical anion (CO2.-) is known to be generated in vivo through various chemical and biochemical pathways. Electron paramagnetic resonance (EPR) spin trapping with the commonly used spin trap, 5,5-dimethyl-1-pyrrolineN-oxide ( DMPO), has been employed in the detection of CO2.-. The thermodynamics of CO2.- addition to DMPO was predicted using density functional theory (DFT) at the B3LYP/6-31+G**//B3LYP/6-31G* and B3LYP/6-311+G* levels with the polarizable continuum model (PCM) to simulate the effect of the bulk dielectric effect of water on the calculated energetics. Three possible products of CO2.- addition to DMPO were predicted: (1) a carboxylate adduct, (2) pyrroline-alcohol and ( 3) DMPO-OH. Experimentally, UV photolysis of H2O2 in the presence of sodium formate (NaHCO2) and DMPO gave an EPR spectrum characteristic of a C-centered carboxylate adduct and is consistent with the theoretically derived hyperfine coupling constants (hfcc). The pKa of the carboxylate adduct was estimated computationally to be 6.4. The mode of CO2.- addition to DMPO is predicted to be governed predominantly by the spin (density) population on the radical, whereas electrostatic effects are not the dominant factor for the formation of the persistent adduct. The thermodynamic behavior of CO2.- in the aqueous phase is predicted to be similar to that of mercapto radical ((SH)-S-.), indicating that formation of CO2.-in biological systems may have an important role in the initiation of oxidative damage in cells.