The proton conducting Ba3Ca1.18Nb1.82O9-delta complex oxide electrolyte is shown to have proton conductivity sigma(H) > 10(-2) S/cm with transport number t(H) > 0.98 at 600 degrees C. Concentration cell measurements up to 800 degrees C with different gases show no sign of residual electronic conduction with a total ionic transport number, t(H) + t(O), that is nearly unity. Further, it is found that only 1/3 of the oxygen-ion vacancies, i.e. those that are replaceable by protons, participate in the conduction process, while the remaining deeply trapped V-O(..) are virtually immobilized. With an electrolyte disc of 1.0 mm thickness, a hydrogen fuel cell operating at 500-600 degrees C is shown to generate an open circuit voltage of 1.0 V and short circuit current density near 130 mA/cm(2), Also, no electrolytic degradation with respect to the cell EMF was detected after operation in O-2 or CO/CO2 gases for more than 100 h, These qualities make the compound an excellent candidate as a solid oxide electrolyte for hydrogen fuel cells, humidity/hydrogen sensors and pumps.