In order to identify competitive ion-conducting materials in ceria-carbonates composite electrolytes, M-NLCO (M = Ce0.8Sm0.2O2-delta (SDC), Ce0.8Gd0.2O2-delta (GDC), Ce0.8Y0.2O2-delta (YDC); NLCO = 0.53Li(2)CO(3)-0.47Na(2)CO(3)) sintered at different temperatures (600 degrees C, 625 degrees C, 650 degrees C, 675 degrees C and 700 degrees C) have been prepared and characterized. It is found that independent of systems, the 675 degrees C-sintered composites in M-NLCO always present the highest conductivities because of the best NLCO distribution and interfacial microstructures. Moreover, among three composites (sintered at 675 degrees C), the total (sigma(t)) and grain boundary (sigma(gb)) conductivities measured at 600 degrees C are ranked as: SDC-NLCO (sigma(600)(t-SDC-NLCO)degrees(C) = 9.1 x 10(-2) Scm(-1), sigma(600)(gb-SDC-NLCO)degrees(C) = 28.1 x 10(-2) Scm(-1)) > GDC-NLCO (sigma(600)(t-SDC-NLCO)degrees(C) = 5.8 x 10(-2) Scm(-1), sigma-(600)(gb-GDC-NLCO)degrees(C) = 18.9 x 10(-2) Scm(-1)) > YDC-NLCO (sigma-(600)(t-YDC-NLCO)degrees(C) = 3.1 x 10(-2)Scm(-1), sigma-(600)(gb-YDC-NLCO)degrees(C) = 12.6 x 10(-2) Scm(-1)), which is attributed to ionic-radius compatibility between the dopant and the host as well as the NLCO distribution and interfacial microstructures. It can be concluded that ionic-radius compatibility between the dopant and the host, NLCO distribution and interfacial microstructures have important effects on improving ionic conductivities for ceria-carbonates composite electrolytes. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.