Polypyrrole composition over carbon-supported palladium nanoparticles (Pd-PPy/C) is successfully performed as electrocatalysts for acid-based fuel cell. This approach demonstrates that PPy modification on a Pd surface remarkably suppresses the deactivation of Pd under acidic conditions. We attempted to determine the influence of PPy on the Pd surface with respect to stability. For this purpose, a potential cycling test and inductively coupled plasma mass spectrometry (ICP-MS) analysis are applied to this system to investigate how PPy affects Pd surface area loss by dissolution and particle growth/agglomeration. The electronic structure of the catalysts is also analyzed with X-ray absorption-near-edge spectroscopy (XANES) and X-ray photoelectron spectroscopy (XPS). The increase in stability upon PPy modification of Pd nanoparticles can be interpreted as a result of the redox properties of PPy polymer itself because the electronic interaction of Pd and PPy doesn't reveal a shifting of the d-band center (epsilon(d)). Consequently, our work may suggest that conjugated polymer-composited metal catalysts are good potential candidate in the development of efficient electrocatalysts because of the possibility of electrochemical role of the conjugated polymer. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.