The thermal diffusivities of molten single carbonates (Li₂CO₃, Na₂CO₃ and K₂CO₃) and their binary and ternary mixtures have been measured in the temperature range up to 1445K by the forced Reyleigh scattering method using a CO₂ laser. The corrosion resistant material for the sample cell employed is sapphire glass plate, which was chosen through experimental evaluations. The accuracy is estimated to be ±11% to ±22%, depending on the measured salts. In comparison with the present results converted into thermal conductivity, the previous experimental data show values up to about three times larger, which may be due to the presence of convection and radiation. It is found that the thermal conductivity of molten single carbonates decreases with increasing molecular weight. It is also found that the temperature dependence of the thermal conductivity is weakly positive.