Activated carbon was modified with mixed phosphotungstic acid (PWA) and phosphomolybdic acid (PMA) by impregnation. Palladium nanoparticles were loaded on the modified supports to prepare Pd/PWA-PMA-C catalyst by liquid reduction method. Uniformly dispersed Pd nanoparticles were prepared and its particle sizes are in the range of 3.36-4.46 nm. The effects of ratios of PWA and PMA on the catalytic activities of formic acid oxidation were investigated. Electrochemical measurements demonstrated that the activity of Pd/PWA-PMA-C catalyst was enhanced as a factor of 2.17 than Pd/C catalysts when the molar ratio between PWA and PMA is 2: 1. A negative shift (41 mV than Pd/C) of CO stripping current peak and improved stability (104 times than Pd/C) were observed for the Pd/W2M1-C catalyst. The amount of decomposed formic acid over the Pd/W2M1-C catalyst is only 17.6% of the Pd/C catalyst, which indicated its excellent inhibition performance of the decomposition of formic acid. In addition, faster charge-transfer kinetics of the formic acid oxidation for the Pd/W2M1-C catalyst was found through electrochemical impedance spectroscopy (EIS) measurements. The better electrocatalytic activity and stability of Pd/W2M1-C catalyst may be attributed to the synergistic effect of the PWA and PMA modification. (C) 2015 The Electrochemical Society. All rights reserved.