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Perovskite-type oxides of Ba x Sr 1 - x FeO 3 (x = 0, 0.1, 0.5, 0.9 and 1.0) were examined as a sensing electrode (SE) material of potentiometric yttria-stabilized zirconia (YSZ)-based oxygen sensors. The sensors using each of Ba x Sr 1 - x FeO 3 -SEs were found to be insensitive to the various examined gases (100 ppm each) such as NH 3 , H 2 , C 3 H 6 , C 3 H 8 , CH 4 , NO 2 , NO and CO, while they were sensitive to oxygen in the concentration range of 0.05-21 vol.% at 600 °C. The number of electron (n) in the electrochemical reaction of oxygen for the present sensors was estimated to be about 4.2 based on Nernst equation in the examined concentration range. The estimated n values for the present sensors are consistent with that (n = 4.2) for the conventional YSZ-based oxygen sensor using Pt-SE, being almost same as the theoretical value (n = 4). The YSZ-based sensor attached with BaFeO 3 -SE further showed excellent responses at temperatures of 450–600 °C. The electric potential difference (Δ V ) value of the present sensor was rather stable under the operating conditions (relative humidity: 20–100%, 400 ppm CO 2 , 600 °C) for the examined period of 14 days. In addition, it was found that the sensor using BaFeO 3 -SE could work well as a lambda (λ) sensor at 600 °C in the combustion gas of O 2 /C 3 H 8 , as is the case with the sensor using Pt-SE. [ABSTRACT FROM AUTHOR]