A mechanism describing the scavenging Of O-2(-) by NO to produce NO3- in the condensed phase is presented. In the proposed mechanism, H2O acts as an overall catalyst. However, it is oxidized temporally by ONOO- to a second intermediate complex consisting of NO2- and H2O2. This complex is subsequently rearranged to hydrated nitrate, [NO3-, H2O]. The structures of the various species in the elementary reactions are investigated by ab initio methods. The thermodynamical data in condensed media are estimated from polarized continuum calculations. Thermodynamic considerations show that the proposed mechanism is feasible. However, the estimates,for the activation energies suggest that the mechanism is not of much physiological importance.