Thermal shock of high density (>99% relative density) zirconium diboride (ZrB2) and zirconium diboride containing 30 vol.% silicon carbide (SiC) was studied. Calculations based on thermal shock theory indicated that ZrB2-30vol.% SiC should have greater thermal shock resistance than ZrB2 due to its higher fracture strength. Water quench tests revealed essentially the same critical thermal shock values for both materials (Delta T-crit = similar to 400 degrees C. A finite element model was used to estimate the temperatures and stresses in both ceramics during quench testing. The model predicted that maximum thermal stresses during the experimental quench test exceeded the strength of ZrB2 (568 MPa) but not ZrB2-30 vol.% SiC (863 MPa). The lower than predicted thermal shock resistance of ZrB2-SiC was attributed to the non-uniform cooling between the ZrB2 matrix grains and the SiC particulate phase. (C) 2008 Elsevier B.V. All rights reserved.