Electrical conductivity and transport properties for composites consisting of samaria-doped ceria (SDC) and lithium-potassium carbonate were investigated at temperature of 450-550 degrees C. The conductivity at temperature above 500 degrees C as measured with ac impedance spectrum is 0.06-0.07 S cm(-1), which is about one magnitude higher than that of SDC. The conductivity increases with carbonate content and is boosted to several times when the volume fraction of carbonate reaches 30%. An abrupt change in the electrical conductivity at 500 degrees C infers that superionic phase transition possibly occurs in the interface between SDC and carbonate. Negligible electronic conduction, similar to 10(-4) S cm(-1) at 500-550 degrees C, is observed with an ion-blocking cell. The dc transport numbers of carbonate ion and proton are determined by carbon dioxide and vapor concentration cells, respectively. Carbonate ionic transport number is about 0.67 above 500 degrees C whereas the protonic transport number is below 0.1. The oxygen ion transport number is calculated to be 0.23 at 550 degrees C and 0.31 at 500 degrees C. (c) 2006 Elsevier Ltd. All rights reserved.