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Abstract. Electroconductivity of BaCeo.9Ndo .IO3__a was studied as a function of the composition of the H2+H20+Ar mixture and temperature in the interval from 873 to 1173 K. It was shown that the electroconductivity was independent of Pea (0.97 to 0.10 atm) and Po: (10 --~ to 10 -26 atm), but depended on Pmo (0.08 to 0.005 atm). A mathematical processing of the Peao dependencies of the electroconductivity, which was performed in terms of a classical model of defect formation in hightemperature proton-conducting solid electrolytes, yielded equilibrium constants of the reaction of water dissolution in BaCe0.gNd0AO_~_a and mobilities of protons and oxygen ions. The temperature dependencies of these quantities were used to determine the mobility activation energies of protons (E a = 34+7 kJ/mole) and oxygen ions (E~ = 72+8 kJ/mole), and also the enthalpy (AH = -150+25 kJ/mole) and the entropy (AS = 153+26 kJ/mole.K) of the reaction of water dissolution in BaCeo oNdo lOs-a-

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