Using oxygen, water-vapor and hydrogen concentration cells and Ba3Ca1.12Nb1.82O9-delta (BCN18) as a solid state electrolyte the Nernstian voltages were measured at elevated temperatures. Proton transport numbers near unity were observed for p(O2) < 10(-5) bar. In the domain of p(O2) > 10(-5) bar the electron hole transport prevailed over the ionic one, e.g. at p(O2) approximate to 0.2 bar the proton transport number reached only values of 0.15. Under fuel cell conditions spanning the whole range of p(O2) over 23 orders of magnitude an ionic transport number of 0.9 was achieved at 700 degrees C. Treatment of BCN18 in hydrogen atmospheres was found to produce a high density of grain boundary cavities leading to pronounced mechanical weakness.