The thermal diffusivity values of La2.7Sm0.3S4 , La2.3Sm0.7S4 , La2.7Eu0.3S4 , La2.2Eu0.8S4 , La2.8Yb0.2S4 , and La2.3Yb0.7S4 were measured by the flash diffusivity method from 400 to 1000 °C. The values ranged from 0.007 to 0.018 cm2 /s. The thermal conductivities of the six ternary rare‐earth sulfides were calculated from the thermal diffusivity data and ranged from 10.7 to 31.6 mW/cm °C. Of the alloys measured, La2.2Eu0.8S4 had the highest thermoelectric figure of merit at 1000 °C (0.52×10-3 °C-1). All of the samples had an oxysulfide present at the grain boundaries which degraded their high‐temperature performance.