BaCe0.4Zr0.4Y0.2O3-delta (BCZY) was synthesized by solid state reaction, calcined and sintered at 1600 degrees C for 12 h. Crystal structure was studied by X-ray diffraction (XRD). Morphology and porosity were determined by scanning electron microscopy (SEM). Crystalline structure, oxygen non-stoichiometry, linear expansion and electrical conductivity were characterized under oxidizing and reducing atmosphere by high temperature X-ray diffraction (HT-XRD), thermogravimetry (TG), dilatometry, and electrochemical impedance spectroscopy (EIS), respectively. Chemical stability under CO2-rich atmosphere was evaluated by TG. BCZY electrical conductivity was studied by EIS under O-2-containing atmosphere with water vapor (2% H2O) and heavy water vapor (2% D2O) in order to evaluate protonic conductivity. Throughout these techniques, interstitial proton incorporation/loss was observed under oxidizing and reducing atmosphere, between 300 and 500 degrees C. The conductivity presents two contributions. The bulk conductivity at high frequencies takes the same value regardless wet oxidizing or reducing atmosphere, decreasing its value in presence of D2O vapor supporting H-conductivity. On the other hand, the grain boundary conductivity was strongly dependent on the nature of wet atmosphere. (C) 2016 Elsevier B.V. All rights reserved.