Concerted proton-electron transfer (CPET) is documented for the homogeneous reduction of O-2 to HO2 center dot in water by the one-electron-reduced heteropolytungstate anion, alpha-PW12O404-(1(1e)). At 0.01-0.3 M H+, O-2 reduction occurs via outer-sphere electron transfer followed by proton transfer (ETPT, with rate constant k(ET)). 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)k(obs) = k(ET) + k(CPET)[H+] (k(ET) = 1.2 M-1 s(-1); k(CPET) = 0.8 M-2 s(-1)). Control experiments rule out preassociation between H+ and 1(1e). 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 O-2 by alpha-SiW12O405- confirm theoretical predictions that CPET decreases in significance as ET becomes less endergonic. Marcus analysis, including the temperature dependence of Delta G degrees, gives reorganization energies, lambda(ET) = 41.5 kcal mol(-1) and lambda(CPET) = 52.4 kcal mol(-1). At 1.5 M H+, similar to 75% of the (1(1e),O-2) encounter pairs form within 6 angstrom of H+ ions. This value (6 +/- 1 angstrom) is the "reaction distance" for proton diffusion and probably close to that for CPET. Even so, the 70-200 ps lifetimes of the (1(1e),O-2) pairs provide additional time for H+ to diffuse closer to O-2. CPET is first-order in [H+] because k(e) for "cage escape" from (1(1e),O-2) pairs is much larger than k(CPET), such that the rate expression for CPET becomes -(1/2)d[1(1e)]/dt = (k(d)/k(e))k(CPET)[1(1e)][O-2][H+], where k(d) is the rate constant for (1(1e),O-2) 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.