The bimetallic, 20 wt % Pt4Mo/C electrocatalyst prepared by the colloidal aluminum-organic reductive- stabilization pathway by the Bonnemann's group (unpublished) was studied by means of X-ray photoelectron spectroscopy (XPS) and electrochemical measurements aiming to a deeper insight on the structure, chemical state, catalytic and electrocatalytic functioning under CO/H-2 mixtures. Combined XPS and Ar+ sputtering experiments revealed that Mo and Pt form bulk alloy nanoparticles, the surface of which is enriched with Pt atoms, while Mo6+(233.2 eV), Mo5+ or Mo4+(231.5 eV) species exist on the nanoparticles' surface. PtMo nanoparticles are shown to be able to electrochemically oxidize CO at very low potential that coincides with the H-2 oxidation potential. This unique property depends on the oxidation state of the Mo surface species and specifically on the reactive OH species of MoO(OH)x and its attribute to spontaneously dissociate H2O. Due to this property PtMo nanoparticles can catalyze the water-gas shift reaction at temperatures as low as 60 C. However, the operation of the electrocatalyst at potentials higher than 0.15-0.2 V, with respect to the H-2 electrode voltage, results in the formation of MoO3 and the deactivation of the low potential CO oxidation sites. (c) 2007 The Electrochemical Society.