Ultrafine ZrB2-SiC composite powders have been synthesized in situ using carbothermal reduction reactions via the sol-gel method at 1500 degrees C for 1 h. The powders synthesized had a relatively smaller average crystallite size (< 200 nm), a larger specific surface area (similar to 20 m(2)/g), and a lower oxygen content (similar to 1.0 wt %). Composites of ZrB2+20 wt% SiC were pressureless sintered to similar to 96.6% theoretical density at 2250 degrees C for 2 h under an argon atmosphere using B4C and Mo as sintering aids. Vickers hardness and flexural strength of the sintered ceramic composites were 13.9 +/- 0.3 GPa and 294 +/- 14 MPa, respectively. The microstructure of the composites revealed that elongated SiC grain dispersed uniformly in the ZrB2 matrix. Oxidation from 1100 degrees to 1600 degrees C for 30 min showed no decrease in strength below 1400 degrees C but considerable decrease in strength with a rapid weight increment was observed above 1500 degrees C. The formation of a protective borosilicate glassy coating appeared at 1400 degrees C and was gradually destroyed in the form of bubble at higher temperatures.