Samaria-doped ceria (SDC)/carbonate composite electrolytes were developed for low-temperature solid oxide fuel cells (SOFCs). SDC powders were prepared by oxalate co-precipitation method and used as the matrix phase. Binary alkaline carbonates were selected as the second phase, including (Li-Na)(2)CO(3), (Li-K)(2)CO(3) and (Na-K)(2)CO(3). AC conductivity measurements showed that the conductivities in air atmosphere depended on the salt composition. A sharp conductivity jump appeared at 475 degrees C and 450 degrees C for SDC/(Li-Na)(2)CO(3) and SDC/(Li-K)(2)CO(3), respectively. However, the conductivities of SDC/(NaK)(2)CO(3) increase linearly with temperature. Single cells based on above composite electrolytes were fabricated by dry-pressing and tested in hydrogen/air at 500-600 degrees C. A maximum power density of 600, 550 and 550 mW cm(-2) at 600 degrees C was achieved with SDC/ (Li-Na)(2)CO(3), SDC/(Li-K)(2)CO(3) and SDC/(Na-K)(2)CO(3) composite electrolyte, respectively, which we attribute to high ionic conductivities of these composite electrolytes in fuel cell atmosphere. We discuss the conduction mechanisms of SDC/carbonate composite electrolytes in different atmospheres according to defect chemistry theory. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.