A new mechanism for the dichloride radical anion (Cl2•-) formation in diluted acidic chloride solutions is proposed on the grounds of pulse radiolysis measurements of the optical absorption growth at 340 nm and the density functional theory and Hartree-Fock computations. We show that the rate of •OH conversion into Cl2•- is determined by the equilibrium concentration of the ionic pair H3O+·Cl-. According to the proposed mechanism, the diffusional encounter of •OH and H3O+·Cl- is followed by fast concerted charge/proton transfer ( k(25 °C) = 6.2 × 1012 s-1) to yield Cl•, which then reacts with Cl- to produce Cl2•-. The mechanism has been confirmed by the observed first-order growth of the Cl2•- absorption and a direct proportionality of the rate constant to the activities of H3O+ and Cl- ions. The salt effect on the rate of Cl2•- formation is due to the ionic strength effect on the equilibrium H3O+ + Cl- ⇄ H3O+·Cl-.