A systematic investigation on the effective ionic conductivity (sigma(m)) of a novel intermediate-temperature mixed oxide-ion and carbonate-ion conductor (MOCC) consisting of a ceria phase and a carbonate phase is reported. The study explicitly shows that the observed remarkable temperature-dependent sigma(m) is primarily the result of softening/melting of the carbonate phase as the physical state of the carbonate phase transforms from solid, softened to molten. Differential scanning calorimetry analysis complements the understanding of the observed electrical behavior by revealing temperatures of melting and solidification in agreement with the onset temperatures of sigma(m). In addition, the measured sigma(m) agrees reasonably well with that simulated by the effective medium percolation theory. Furthermore, sigma(m) of the MOCC is independent of atmosphere at t < 600 degrees C, exhibiting the characteristics of a good electrolyte. The MOCC-LiNiOx and MOCC-Ni composites are better cathode and anode materials than noble metals for MOCC-based fuel cells, respectively. Finally, no sign of sigma(m) degradation measured at 600 degrees C in air is found over a 3-day period. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3524477] All rights reserved.