An evaluation of over 50 polyoxometalates (POMs) identified the complex NaH3[SiW11Ce(IV)O39] (NaH3(1)) as a selective and effective catalyst for the aerobic oxidation of formaldehyde to formic acid under very mild (including ambient) conditions. 183W NMR, UV-vis, cyclic voltammetry, and potentiometric titration establish that the catalyst is a monomer (Cs symmetry), 1, in solution, while X-ray crystallography (a = 12.9455(15) A, b = 13.2257(16) A, c = 14.5288(17) A, alpha = 81.408(2) degrees , beta = 85.618(2) degrees , gamma = 80.726(2) degrees , P, Z = 1, R1 = 5.79% based on 17244 independent reflections) and IR establish it to be a dimer (Ci 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 O2 pressure, presence of titratable H2O2 and HCO3H intermediates, and inhibition by conventional radical scavengers, all indicate the O2-based oxidations proceed by complex homolytic chemistry (autoxidation and Haber-Weiss radical-chain processes) likely initiated by protonated 1.