The synthesis and characterization (physical and electrochemical) of novel PdAu/Vulcan XC 72 electrocatalysts are described. The catalysts were prepared via deposition and activation of preformed bimetallic colloidal precursors. Physical characterization of the catalysts involved high-resolution transmission electron microscopy and powder X-ray diffraction. Electrochemically, the PdAu-catalysts were characterized by means of the thin-film rotating disk method and by differential electrochemical mass spectrometry. H-2, CO and CO/H-2 oxidations on PdAu-Vulcan electrodes with different hull; compositions were used as probe reactions to determine the CO tolerance of this catalyst system and its physical origin. We propose that the GO-tolerance mechanism at low overpotentials for PdAu/Vulcan electrodes is governed by both a lower CO steady-state surface coverage compared to PtRu/Vulcan due to a lower CO adsorption energy and finite CO oxidation rates, which both result in free surface sites for H-2 oxidation to take place, At elevated temperatures (60 degreesC) this effect is mole pronounced. providing more free active Pd sites for H-2 oxidation. CO/H-2 oxidation measurements (1000 and 250 ppm CO) showed the superior activity of PdAu compared with a state-of-the-art PtRu catalyst at technically relevant potentials, especially ill Pd-rich alloys,