The thermal conductivity, thermal expansion, Youngs Modulus, flexural strength, and brittle–plastic deformation transition temperature were determined for HfB 2, HfC 0·98, HfC 0·67, and HfN 0·92 ceramics. The oxidation resistance of ceramics in the ZrB 2–ZrC–SiC system was characterized as a function of composition and processing technique. The thermal conductivity of HfB 2 exceeded that of the other materials by a factor of 5 at room temperature and by a factor of 2·5 at 820°C. The transition temperature of HfC exhibited a strong stoichiometry dependence, decreasing from 2200°C for HfC 0·98 to 1100°C for HfC 0·67 ceramics. The transition temperature of HfB 2 was 1100°C. The ZrB 2/ZrC/SiC ceramics were prepared from mixtures of Zr (or ZrC), SiB 4, and C using displacement reactions. The ceramics with ZrB 2 as a predominant phase had high oxidation resistance up to 1500°C compared to pure ZrB 2 and ZrC ceramics. The ceramics with ZrB 2/SiC molar ratio of 2 (25 vol% SiC), containing little or no ZrC, were the most oxidation resistant.