Thermal and elastic properties were measured for ceramics that contained as much as 96 wt% zeta phase tantalum carbide (ζ-Ta4C3-x). The ceramics were produced from tantalum hydride that was milled to reduce particle size and then blended with carbon. Powders were reaction hot-pressed at 1800°C for 2 hours under a flowing He environment, which resulted in ζ-Ta4C3-x that was about 99% dense. The main secondary phases present in the reacted ceramic were TaC and Ta2O5. ζ-Ta4C3-x had a thermal conductivity of 9.6 W/m·K and an electrical resistivity of 160 ± 4.2Â µΩ-cm, which are lower and higher than those of TaC, respectively. The Young's modulus was 379 ± 5 GPa and the hardness was 5.1 ± 0.7 GPa, which are also both lower than TaC. This study is the first to report the thermal properties and elastic moduli of high-purity ζ-Ta4C3-x.