Abstract The structures and properties of the O 2+O − 2 electron-transfer system in the quartet state are studied in this paper at the UMP2(full)/6-311+G* basis set level, encompassing five selected coupling structures: two T-type, collinear, parallel and crossing. It is based on the golden rule of the time-dependent perturbation theory. The structures, potential-energy surfaces and relevant energy indexes of these encounter complexes are obtained. The coupling interaction between the acceptor and donor has also been calculated by a new method and is indicated as an important factor in determining the electron-transfer rate. This negatively charged encounter complex is in a high-spin state, has larger contact distance (∼3 Å) and weak interaction, and is structurally unstable, not easily observed experimentally. Thus high-quality treatments with electron correlation and large basis sets are needed in the theoretical analysis. The electronic transmission factor for the reacting system O 2+O − 2 is less than unity (ca. 0.01–0.6), thus the electron-transfer reaction is nonadiabatic in nature. Among the five selected transition-state structures, electron transfer is more likely to take place via the T-type and P-type structures. The electron-transfer rate for this system is generally ca. 10 11 M −1 s −1 in the gas-phase process.