AC impedance spectroscopy has been performed on 40 mol% gadolinia-doped ceria electrodes on yttria stabilized zirconia (YSZ) at 700-950 degrees C in reducing CO/CO2 atmospheres. Area-specific-resistance (ASR) values for this electrode were in the range of 0.8-37 Omega-cm(2), about two orders of magnitude lower than measurements on Pt electrodes and slightly lower than data on Ni-YSZ electrodes in the literature under similar temperature and partial pressure of oxygen (P-O2) conditions. A continuum-based model of this electrode is described and ail analysis performed to extract the vacancy diffusion coefficient (D-nu) and surface exchange rate coefficient (R-0) as a function of temperature and P-O2, from the impedance results. The D-nu data agree reasonably well with published measurements of the tracer diffusion coefficient (D*) based on isotope profiling by secondary ion mass spectroscopy (SIMS) and conductivity measurements on 40 mol% GDC. The R-0 values are a factor of 3 lower than the published measurements of the surface reaction rate (k) obtained from isothermal thermogravimetric relaxation, and decrease with increasing P-O2. Values of the thermodynamic factor (A) calculated from the fitted model parameters matched well with those calculated from oxygen non-stoichiometry data in the literature. Published by Elsevier B.V.