Perovskite-type BaMg0.33Nb0.34Fe0.33O3 (-) (delta) (BMNF) and double perovskite-type Ba2Ca0.66Nb0.68Fe0.66O6 (-) (delta) (BCNF) were prepared by conventional solid-state reaction at 1400 degrees C in air. Ex-situ powder X-ray diffraction (PXRD) and high temperature PXRD (30-700 degrees C) of BCNF and BMNF under CO2 and SO2 exhibited excellent chemical stability. Chemical stability under SO2 was further confirmed by scanning electron microscopy (SEM) combined with energy dispersive X-ray analysis (EDX), where no sulfur-containing compounds were detected within the detection limit of the equipment BCNF and BMNF were tested for their sensing properties in CO2 and SO2 at 500-700 degrees C by applying constant DC voltage (0.1V). Ag and Pt current collectors were employed as electrodes for BCNF and showed similar response at 600 degrees C (t(90) = 7 min) for ppm level of CO2 in dry synthetic air. BCNF sensor was found highly stable and very selective toward CO2 in presence of SO2 (0-20 PPm) (C) 2013 Elsevier B.V. All rights reserved.