A selection of eighteen different heteropolyacids (HPAs) were adsorbed onto Pt-on-carbon catalysts and investigated for enhanced proton exchange membrane fuel cell (PEMFC) anode CO tolerance. Structures tested were Keggin, Wells-Dawson, and a "Wells-Dawson sandwich." Compositions were varied by replacing W or Mo addenda atoms with V atoms in the Keggin structures and W addenda atoms with Fe, Mn, Ni, Co, and Cu in the Wells-Dawson or Wells-Dawson sandwich structures. Of those tested, it was found that only Keggin-structured HPAs of composition H3+xPMo12-xVxO40 where V = 0, 2, and 3 had significant activity toward CO oxidation. This performance increased with V content. Using fuel poisoned with 100 ppm CO the addition of H6PMo9V3O40 improved the current obtained at 0.5 V to 525 from 229 mA/cm(2) in fuel cell polarization experiments. Anode polarization experiments showed a performance improvement of about 0.360 V with the addition of this same HPA. These HPAs were behaving as catalysts and were not simply improving the resistance of mass transport of the electrodes. Cyclic voltammetry demonstrated that HPAs exhibit reversible reductions and oxidations in the fuel cell electrode under PEMFC operating conditions. CO tolerance increases were only noticed for HPAs which were reduced and oxidized at low overpotentials. (c) 2007 The Electrochemical Society.