This work explores the formation of undesirable electronic leakage in BaCe,Zr0.9 xYo.103 a (BCZY) proton conducting ceramic thin membranes used in H2O-electrolysis and hydrogen -pumping applications. Very low hydrogen fluxes (hydrogen transport number below 0.4) through thin BCZY membranes during electrolysis operation were recently reported. These poor performances were explained by the presence of an undesirable electronic leakage that forms due to reduction of cerium in the BCZY material. In this paper, we investigate the influence of the cerium content in the BCZY membranes on the magnitude of this electronic leakage. Three compositions were fabricated with x = 0, 0.1, and 0.2 (BZY10, BCZY18, and BCZY27, respectively). Unique gas conditions were chosen to isolate specific charge carriers present in the thin membranes, and decouple the electronic -conduction effects. While significant electronic leakage was observed in the BCZY27 material under these well-defined conditions, minimal leakage was detected in BCZY18 and BZY10. These results confirm that the presence of the electronic leakage is linked to the cerium content and is most probably due to the reduction of cerium ions under bias. Additionally, BCZY18 is the preferred composition for electrolysis applications due to its relative lack of electronic conductivity, reasonable protonic conductivity, and affinity for high -density sintering. (C) 2016 Elsevier B.V. All rights reserved.