Low-temperature CO oxidation over a compositional series of Pd-Au nanoalloy catalysts supported on silica fume was studied. Except for the pure metals, these materials invariably showed biphasic separation into palladium- and gold-rich components. Performance was optimal for a catalyst of bulk composition Pd4Au1, a mixture of Pd90Au10 (72.5 at. %) and Pd31Au69 (27.5 at. %), that was remarkably active at 300 K and more stable than a pure Au catalyst. For bulk materials dominated by Pd (Pd:Au = 16:1; 8:1; 4:1), the palladium-rich alloy fraction frequently adopted hollow sphere or annular morphology, while the gold-rich crystals were often multiply twinned. Quantitative powder X-ray diffraction (XRD) showed that under the synthesis conditions used, the Au solubility limit in Pd crystals was similar to 12 at. %, while Pd was more soluble in Au (similar to 31 at. %). This was consistent with X-ray photoelectron spectroscopy (XPS), which revealed that the surfaces of Pd-rich alloys were enriched in gold relative to the bulk composition. In situ Fourier transform infrared spectra collected during CO oxidation contained a new band at 2114 cm(-1) (attributed to linear CO-Au/Au-Pd bonds) and reduced intensity of a band at 2090 cm(-1) (arising from a linear CO-Pd bond) with escalating Au content, indicating that the Pd sites became increasingly obscured by Au. High-resolution electron micrographs (HRTEM) of the Pd-rich alloys revealed atomic scale surface defects consistent with this interpretation. These results demonstrate that gold-containing biphasic Pd nanoalloys may be highly durable alternatives for a range of catalytic reactions.