The thermal conductivity of composite materials, specifically alkali carbonate-based melts filled with fine alpha-Al2O3, was measured in relation to the temperature and thickening agent content using the steady-state coaxial cylinder method. The thermal conductivity was found to be a linear function of the temperature in the range from 670 K to 930 K for salt melts and all of the thickened melt-oxide mixtures that were studied, while this value increased non-linearly as the alpha-Al2O3 volume fraction increased showing a tendency to be unchanged at alpha-Al2O3 volume concentrations greater than 15%. The thermal conductivity of the (Li2CO3 - Na2CO3 - K2CO3)(eut) + alpha-Al2O3 composites measured was adequately described by the Maxwell-Wagner equations derived for two-phase heterogeneous materials. The alternating sign of the temperature coefficients of the composites thermal conductivity and peculiarities of its concentration dependency, point to the existence of "ionic heat conductor - oxide heat insulator" transaction in alkali carbonate-based melts thickened with fine alpha-Al2O3. (C) 2017 Elsevier Ltd. All rights reserved.