- Concerted proton−electron transfer (CPET) is documented for the homogeneous reduction of O2 to HO2• in water by the one-electron-reduced heteropolytungstate anion, α-PW12O404− (11e). At 0.01−0.3 M H+, O2 reduction occurs via outer-sphere electron transfer followed by proton transfer (ETPT, with rate constant kET). Between 0.30 and 1.9 M H+, rates increase linearly with [H+] due to a parallel CPET pathway in which H+ is now a reactant: (1/2)kobs = kET + kCPET[H+] (kET = 1.2 M−1 s−1; kCPET = 0.8 M−2 s−1). Control experiments rule out preassociation between H+ and 11e. Analysis of plausible rate expressions shows that the first-order dependence on [H+] is uniquely consistent with multisite CPET, and a deuterium kinetic isotope effect of 1.7 is observed. Reductions of O2 by α-SiW12O405− confirm theoretical predictions that CPET decreases in significance as ET becomes less endergonic. Marcus analysis, including the temperature dependence of ΔG°, gives reorganization energies, λET = 41.5 kcal mol−1 and λCPET = 52.4 kcal mol−1. At 1.5 M H+, ∼75% of the (11e,O2) encounter pairs form within 6 Å of H+ ions. This value (6 ± 1 Å) is the “reaction distance” for proton diffusion and probably close to that for CPET. Even so, the 70−200 ps lifetimes of the (11e,O2) pairs provide additional time for H+ to diffuse closer to O2. CPET is first-order in [H+] because ke for “cage escape” from (11e,O2) pairs is much larger than kCPET, such that the rate expression for CPET becomes −(1/2)d[11e]/dt = (kd/ke)kCPET[11e][O2][H+], where kd is the rate constant for (11e,O2) pair formation. Overall, the findings suggest that the emergence of CPET, with hydronium ion as the proton donor, may prove a general feature of sufficiently endergonic reductions of dioxygen by otherwise “outer-sphere” complexes (or electrode reactions) at sufficiently low pH values in water.