Carbon supported Pd-Pt electrocatalysts (Pd-Pt/C) with low Pt content were investigated in proton exchange membrane fuel cells (PEMFCs) with pure H-2 and CO/H-2 as the feeding fuels, respectively. The Pd-Pt/C catalysts showed high activity for hydrogen oxidation reaction (HOR) and improved CO tolerance. Electrochemical impedance spectroscopy (EIS) was employed to probe the in-situ information of the improved CO tolerance. The dependence of Nyquist plots and Bode plots on current density and feeding gas was investigated in low polarization region. The results of EIS analysis indicated that the improved CO tolerance of Pd-Pt/C catalysts can be attributed to the lower coverage of CO on the Pd-Pt bimetal than that on the pure Pt. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.