The electrochemical properties of the metal vertical bar doped ceria vertical bar metal system have been examined by A.C. impedance spectroscopy at a range of temperatures (575-650 degrees C) and oxygen partial pressures (8 x 10(-26)-0.21 atm). Under reducing conditions, the inverse of the interfacial resistances for both Pt and Au electrodes on samarium-doped ceria (SDC) displayed a p(O2) (-1/4) dependence on oxygen partial pressure and an Arrhenius dependence on temperature with an activation energy of similar to 2.7 eV. The similarity in the characteristics of the two types of electrode materials indicates that the electrochemical oxidation of hydrogen occurs predominantly at the ceria vertical bar gas interface. Moreover, the behavior, which is similar to that of the electronic conductivity of SDC, is consistent with the hypothesis that electron migration from the reaction site to the metal current collector is the rate-limiting step. (c) 2008 Elsevier B.V. All rights reserved.