The carbon-supported Pd-Au catalysts (Pd-Au/C) with different alloying degree are prepared in the aqueous solution with and without tetrahydrofuran (THF) by a chemical reduction method. The studies of X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM) measurements show that the alloying degree of the Pd-Au/C catalyst prepared in the aqueous solution is much lower than that of Pd-Au/C catalyst prepared in the H2O/tetrahydrofuran (THF) mixture solution, indicating the presence of THF can obviously enhance the alloying degree of Pd-Au nanoparticles during the preparation of Pd-Au/C catalyst. The electrochemical measurements illustrate the electrocatalytic activity of Pd-Au/C catalyst for the formic acid electrooxidation is strongly dependent on alloying degree of Pd-Au nanoparticles. The Pd-Au/C catalyst with high alloying degree shows a higher electrocatalytic activity and stability for the formic acid electrooxidation compared to the Pd-Au/C catalyst with low alloying degree, which can be ascribed to enhancement of CO tolerance and possible suppression of dehydration pathway in the course of formic acid electrooxidation. (C) 2011 Elsevier B.V. All rights reserved.