An evaluation of over 50 polyoxometalates (POMs) identified the complex NaH3 [(SiW11CeO39)-O-IV] (NaH(3)1) as a selective and effective catalyst for the aerobic oxidation of formaldehyde to formic acid under very mild (including ambient) conditions. W-183 NMR, UV-vis, cyclic voltammetry, and potentiometric titration establish that the catalyst is a monomer (C-s symmetry), 1, in solution, while X-ray crystallography (a = 12.9455(15) Angstrom, b = 13.2257(16) Angstrom, c = 14.5288(17) Angstrom, alpha = 81.408(2)degrees, beta = 85.618(2)degrees, gamma = 80.726(2)degrees, P1 P(1) over bar, Z = 1, R1 = 5.79% based on 17244 independent reflections) and IR establish it to be a dimer (C-i symmetry), 1(2), in the solid state. Several lines of evidence, including the parabolic kinetic order in 1, nonlinear Arrhenius plot, independence of the rate on O-2 pressure, presence of titratable H2O2 and HCO3H intermediates, and inhibition by conventional radical scavengers, all indicate the O-2-based oxidations proceed by complex homolytic chemistry (autoxidation and Haber-Weiss radical-chain processes) likely initiated by protonated 1.