The thermal properties of zirconium diboride (ZrB2) ceramics with carbon additions of up to 3 wt% were characterized up to 2000 degrees C. Carbon contents were selected to produce ZrB2 that was nominally pure, contained dissolved carbon, or contained carbon inclusions. The microstructure and density changes that resulted from the carbon additions affected the thermal behavior of ZrB2 at room and elevated temperatures. Thermal diffusivity at 200 degrees C increased from 0.150 cm2/s for nominally pure ZrB2 to 0.175 cm2/s for ZrB2 with 3 wt% carbon. The thermal diffusivity decreased with increasing temperature, reaching a value of 0.143 cm2/s at 2000 degrees C for ZrB2 with 3 wt% carbon. In addition, thermal diffusivity changed irreversibly during the first thermal cycle after densification due to changes in the microstructure that started between 1550 degrees C and 1650 degrees C. Heating resulted in the formation of a new phase, growth of ZrB2 grains, changes in the morphology of carbon inclusions, and migration of W impurities from the ZrB2 matrix into the new phase. Heat capacity, unlike thermal diffusivity, did not change during thermal cycling. Thermal conductivity, which was calculated from thermal diffusivity, heat capacity, and density, was as high as 64.2 W.(m.K)-1 at 2000 degrees C for ZrB2 with 3 wt% carbon. The phonon contribution to thermal conductivity decreased to nearly zero with the addition of 3 wt% carbon due to the presence of elongated carbon inclusions around ZrB2 grains.