B-site substituted ordered double perovskite-type structure BaCa0.33Nb0.67-xFexO3-delta (x = 0.00, 0.17, 0.25, 0.50) (BCNF) was synthesized by conventional solid state method at 1400 degrees C in air. In-situ high temperature powder X-ray diffraction (HT-PXRD) under CO2 displayed a single-phase double perovskite-type structure at 25-800 degrees C, suggesting excellent chemical stability. Selected area electron diffraction (SAED) further supported the PXRD. Physical (e. g., density) and electrical properties (e. g., conductivity, sensitivity) were correlated with amount of Fe content in BCNF. BCNF changed its impedance under CO2 (ppm level) mixed with dry synthetic air (21% CO2 in N-2) at 500 degrees C and 700 degrees C. BCNF with the highest Fe-doped, BaCa0.33Nb0.17Fe0.50O3-delta (BCNF50), exhibited an excellent sensitivity (t(90) similar to 7 min at 500 degrees C and t(90) similar to 4 min at 700 degrees C) comparing to lower Fe-content BCNF samples. The sensor response (current density) as a function of log CO2 showed a linear behavior. The critical role of Fe in mixed conducting BCNF has been investigated to illustrate its potential applications for real time detection of CO2 in ppm level. (C) 2013 The Electrochemical Society.