BaCe0·7Zr0·1Y0·16Zn0·04O3-δ perovskite has been investigated due to its potential as an electrolyte in industrial steam electrolysis applications. The lowest area specific resistance (ASR) is achieved as 4.0 Ω cm2 at 711 °C under 3% wet Ar atmosphere. The conductivity is calculated as 2.93 × 10−2 S cm−1 and kept stable for a ~70 h testing period. ASR increased at lower temperature (511 °C) under the same atmosphere and a new impedance arc (with 4.5 Ω cm2 ASR and 2 × 10−8 F equivalent capacitance) is formed, indicating second phase formation. No second phase formation is observed at the same temperature under dry 5% H2 in Ar. The second phase formation/degradation of the electrolyte is attributed to Ba(OH)2 and CeO2 formations around 500 °C under wet atmospheres. At elevated temperatures, ~700 °C, BaCe0·7Zr0·1Y0·16Zn0·04O3-δ exhibits both excellent protonic conductivity and stability which makes it a great candidate for both industrial fuel cells and steam electrolysers.