Using oxygen, water-vapor and hydrogen concentration cells and Ba 3 Ca 1 . 1 2 Nb 1 . 8 2 O 9 - δ (BCN18) as a solid state electrolyte the Nernstian voltages were measured at elevated temperatures. Proton transport numbers near unity were observed for p O 2 <10 - 5 bar. In the domain of p O 2 >10 - 5 bar the electron hole transport prevailed over the ionic one, e.g. at p O 2 ~0.2 bar the proton transport number reached only values of 0.15. Under fuel cell conditions spanning the whole range of p O 2 over 23 orders of magnitude an ionic transport number of 0.9 was achieved at 700 o C. Treatment of BCN18 in hydrogen atmospheres was found to produce a high density of grain boundary cavities leading to pronounced mechanical weakness.