The mechanical properties of hot pressed zirconium diboride-silicon carbide-boron carbide (ZrB2-SiC-B4C) ceramics were characterized from room temperature up to 2200⁰C in an argon atmosphere. The average ZrB2 grain size was 3.0μm. The SiC particles segregated into clusters, and the largest clusters were > 30μm in diameter. The room temperature flexural strength was 700MPa, decreasing to 540MPa at 1800⁰C and to 260MPa at 2200⁰C. The strength was controlled by the SiC cluster size up to 1800⁰C. At higher temperatures, strength was controlled by formation of liquid phases, and precipitation of large BN and B-O-C-N inclusions. The mechanical behavior of these materials changes at ~1800⁰C, meaning that extrapolation of properties from lower temperatures is not accurate. Mechanical behavior in the ultra-high temperature regime was dominated by impurities and changes in microstructure. Therefore, the use of higher purity materials could lead to significant improvements in ultra-high temperature strength.