<p>It was formerly demonstrated that O<sub>2</sub>SOO<sup>−</sup> forms at collisions rate in the gas-phase as a result of SO<sub>2</sub> reaction with O<sub>2</sub><sup>−</sup>. Hereby, we present a theoretical investigation of the chemical fate of O<sub>2</sub>SOO<sup>−</sup> by reaction with O<sub>3</sub> in the gas-phase, based on <i>ab initio</i> calculations. Two main mechanisms were found for the title reaction, with fundamentally different products: (i) formation of a van der Waals complex followed by electron transfer and further decomposition to O<sub>2</sub> + SO<sub>2</sub> + O<sub>3</sub><sup>−</sup> and (ii) formation of a molecular complex from O<sub>2</sub> switching by O<sub>3</sub>, followed by SO<sub>2</sub> oxidation to SO<sub>3</sub><sup>−</sup> within the complex. Both reactions are exergonic, but separated by relatively low energy barriers. The products in the former mechanism would likely initiate other SO<sub>2</sub> oxidations as shown in previous studies, whereas the latter mechanism closes a path wherein SO<sub>2</sub> is oxidized to SO<sub>3</sub><sup>−</sup>. The latter reaction is atmospherically relevant since it forms the SO<sub>3</sub><sup>−</sup> ion, hereby closing the SO<sub>2</sub> oxidation path initiated by O<sub>2</sub><sup>−</sup>. The main atmospheric fate of SO<sub>3</sub><sup>−</sup> is nothing but sulfate formation. Exploration of the reactions kinetics indicates that the path of reaction (ii) is highly facilitated by humidity. For this path, we found an overall rate constant of 4.0 × 10<sup>−11</sup> cm<sup>3</sup> molecule<sup>−1</sup> s<sup>−1</sup> at 298 K and 50 % relative humidity. The title reaction provides a new mechanism for sulfate formation from ion-induced SO<sub>2</sub> oxidation in the gas-phase and highlights the importance of including such mechanism in modelling sulfate-based aerosol formation rates.</p>