Electrical conduction in sintered BaCe0.95Ca0.05O2.95 has been investigated at high temperatures (300 < T/degrees C < 1000) in moist N-2, O-2 and H-2 atmospheres via ac impedance spectroscopy. Conduction arises from charge carriers which participate in equilibria with the various atmospheres. At low temperatures (T < 480 degrees C), the conductivity is the same in all atmospheres, protons generated by reaction with water being the dominant charge carriers. At higher temperatures, conductivity is higher in hydrogen than in nitrogen, additional protons being generated by reaction of H-2 with positive holes. In oxygen the material functions as a mixed (oxide + proton + electron) conductor at high temperatures.