Dense solid electrolytes based on lanthanum silicate, La10Si6O27 (LSO) were fabricated and their electrical conducting properties were studied in detail under various atmospheres, including H2O- or D2O-humidified argon. The family showed almost P(O-2)-independent conductivity, while a slight increase in conductivity with increasing P(O-2) Was observed in the high temperature region. The ionic transport number evaluated from the O-2/Ar oxygen concentration cell was higher than 0.964 at 1073-1273 K. The current-voltage characteristic of the fuel cell employing the dense LSO electrolyte was also investigated. (c) 2008 Elsevier B.V. All rights reserved.