Ceria-based composites (ranging from 30 to 70 vol%) including a mixture of Li and Na carbonates (1:2 M ratio, respectively) were sequentially exposed to pure CO2, O-2 and a mixture of 10 vol% H-2 diluted in N-2, at temperatures ranging from 300 to 580 degrees C, to study the role of composite/gas interaction on electrical performance. Impedance spectroscopy measurements performed under these working conditions were complemented by microstructural and structural characterization. Changes in conductivity and electrode impedance when changing the atmosphere from CO2 to O-2 and to H-2 confirmed changes in concentration and/or in mobility of charge carriers and electroactive species. Novel species identified either by X-ray diffraction (hydrated carbonates, only present at the surface of the samples), or by infrared spectroscopy (hydrogen carbonate ions), might be related to the observed conductivity enhancement under H-2. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.