This work provides a detailed electrochemical impedance study for formic acid electro-oxidation on size-controlled Pd/C nanoparticles, the synthesis of which was done by a simple protocol using ethylene glycol as a reducing agent. By controlling KOH concentration, this strategy provides a synthesis method for Pd nanoparticles with a selective size range of 3.9-7.5 nm. The as-prepared Pd nanoparticles exhibited size-dependent electrochemical property and electrochemical characterizations of four different Pd/C nanocatalysts (3.9, 5.2, 6.1, and 7.5 nm) showed that I'd particle with average size of 6.1 nm has the highest formic acid oxidation activity. Electrochemical impedance-based characterizations of formic acid oxidation on Pd/C suggested that at high potentials the adsorbed oxygen species could block the catalyst surface and inhibit the oxidation reaction, as reflected by the negative polarization resistance. Unlike Pd/C, the intermediate adsorbed CO species (COads) plays a critical role for formic oxidation on Pt/C and thus the impedance spectra of Pd/C and Pt/C appear different potential-dependent patterns in the second quadrant. The issue of CO was investigated by an impedance investigation of Pd/C in a mixture of formic acid containing dissolved CO. (C) 2009 Elsevier Ltd. All rights reserved.