The mechanical properties of a ZrB2- 10 vol% ZrC ceramic were measured up to 2300 degrees C in an argon atmosphere. Dense billets of ZrB2- 9.5 vol% ZrC- 0.1 vol% C were produced by hot- pressing at 1900 degrees C. The ZrB2 grain size was 4.9 mu m and ZrC cluster size was 1.8 mu m. Flexure strength was 695 MPa at ambient, decreasing to 300 MPa at 1600 degrees C, increasing to 345 MPa at 1800 degrees C and 2000 degrees C, and then decreasing to 290 MPa at 2200 degrees C and 2300 degrees C. Fracture toughness was 4.8 MPa m(1/2) at room temperature, decreasing to 3.4 MPa m(1/2) at 1400 degrees C, increasing to 4.5 MPa m(1/2) at 1800 degrees C, and decreasing to 3.6 MPa m(1/2) at 2300 degrees C. Elastic modulus calculated from the crosshead displacement was estimated to be 505 GPa at ambient, relatively unchanging to 1200 degrees C, then decreasing linearly to 385 GPa at 1600 degrees C, more slowly to 345 GPa at 2000 degrees C, and then more rapidly to 260 GPa at 2300 degrees C. Surface flaws resulting from machining damage were the critical flaw up to 1400 degrees C. Above 1400 degrees C, plasticity reduced the stress at the crack tip and the surface flaws experienced subcritical crack growth. Above 2000 degrees C, microvoid coalescence ahead of the crack tip caused failure.