The thermal diffusivity of hot-pressed boron carbide pellets was measured at temperatures between 16 and 1400 deg C using a ''flash'' technique. Thermal-conductivity values were calculated from the diffusivity measurements. Empirical porosity correction factors were determined for both properties. Measurements on pellets irradiated at temperatures between 500 and 870 deg C showed significant reduction of both the thermal diffusivity and conductivity. The irradiation-induced decreases saturated rapidly (at boron depletion levels of ~5 x 10/sup 20/ captures/cm/sup 3/) and were greatest in the pellets exposed at the lowest temperature. The losses are attributed to lattice damage in the boron carbide, principally the formation of defects that serve as phonon scattering centers. (auth)