Metal oxide cluster-anion (polyoxometalate, or POM) protecting ligands, [alpha-PW11O39](7-) (1), modify the rates at which 14 nm gold nanoparticles (Au NPs) catalyze an important model reaction, the aerobic (02) oxidation of CO to CO2 in water. At 20 degrees C and pH 6.2, the following stoichiometry was observed: CO + O-2 + H2O = CO2 + H2O2. After control experiments verified that the H2O2 product was sufficiently stable and did not react with 1 under turnover conditions, quantitative analysis of H2O2 was used to monitor the rates of CO oxidation, which increased linearly with the percent coverage of the Au NPs by 1 (0-64% coverage, with the latter value corresponding to 211 +/- 19 surface-bound molecules of 1). X-ray photoelectron spectroscopy of Au NPs protected by a series of POM ligands (K+ salts): 1, the Wells-Dawson ion [alpha-P2W18O62](6-) (2) and the monodefect Keggin anion [alpha-SiW11O39](8-) (3) revealed that binding energies of electrons in the Au 4f(7/2) and 4f(5/2) atomic orbitals decreased as a linear function of the POM charge and percent coverage of Au NPs, providing a direct correlation between the electronic effects of the POMs bound to the surfaces of the Au NPs and the rates of CO oxidation by O-2. Additional data show that this effect is not limited to POMs but occurs, albeit to a lesser extent, when common anions capable of binding to Au-NP surfaces, such as citrate or phosphate, are present.