The electrical conductivity and melting behavior of alkali molten carbonates, Li2CO3 and K2CO3, coexisting with oxide particles, gamma-LiAlO2 and MgO, have been investigated. The conductivity was measured by the ac impedance method. The melting of carbonates caused an abrupt change in the electrical conductivity at the transition point, T-t. The value of T-t decreased with decreasing melt content in the heterophase system. The activation energy, DeltaE(a), increased with a decrease of apparent average thickness of the liquid layer. Differential thermal analysis measurements of gamma-LiAlO2/Li2CO3 coexisting systems showed a shift toward lower temperatures for the endothermic peak of fusion of Li2CO3. The molar enthalpy of fusion, DeltaH(m), of the coexisting phase decreased with decreasing apparent average thickness. A correlation between the transition point, Tt, of the electrical conductivity and the melting point of the thermal analysis, T-m, was found for inorganic powder/Li2CO3 coexisting systems. These variations in the properties of carbonates are attributed to the effect of the solid phase. The influence of gamma-LiAlO2 powder on both transport and thermal properties of molten carbonates is greater than that of MgO particles. (C) 2004 The Electrochemical Society.