A systematic investigation on the effective ionic conductivity (σm) of a novel intermediate temperature mixed oxide-ion and carbonate-ion conductor (MOCC) consisting of a ceria and a carbonate is reported. The study explicitly shows that the observed remarkable temperature-dependent σm 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. DSC analysis complements the understanding of the observed electrical behavior by revealing temperatures of melting and solidification in agreement with the onset temperatures of σm. In addition, the measured σm agrees reasonably well with those simulated by the Effective Medium Percolation Theory (EMPT). Furthermore, σm of the MOCC is independent of atmosphere at t<600oC, exhibiting the characteristics of a good electrolyte. Finally, no sign of σm degradation measured at 600oC in air is found over a three-day period.