Stability under atmospheres containing CO2 and chemical compatibility of GdBaCo2O5+x with different oxide-ion conductors (8 mol% Y2O3-ZrO2, La0.8Sr0.2Ga0.8Mg0.2O3-delta and Ce0.9Gd0.1O1.95) have been studied in order to evaluate its suitability as a cathode material for Solid Oxide Fuel Cells (SOFCs). Thermal evolution of X-ray diffraction patterns of GdBaCo2O5+x in atmospheres with different contents of carbon dioxide (500 ppm to 100% CO) was used to determine the stability of this compound, which is susceptible to carbonate due to the presence of Ba. Remarkable stability under air conditions for long periods of time (100 h) at different temperatures of interest (500, 600 and 700 degrees C) was observed. Chemical compatibility with different electrolytes was evaluated analysing the evolution with calcination temperature (800-1200 degrees C) of the X-ray diffraction patterns of intimate mixtures of GdBaCo2O5+x with each electrolyte. Electrochemical characterization by means of impedance spectroscopy as a part of optimized symmetrical cells (GdBaCo2O5+x/electrolyte/GdBaCo2O5+x) were carried out yielding excellent performance for La0.8Sr0.2Ga0.8Mg0.2O3-delta and Ce0.9Gd0.1O1.95 electrolyte-based cells. (C) 2008 Elsevier B.V. All rights reserved.