Oxidation resistance tests were carried out on HfB2-20 vol.% SiC prepared by spark plasma sintering. The dense samples were exposed from 1400 to 2000 A degrees C in an ambient atmosphere for 1 h. For comparison, the same material was tested using an arc jet to simulate an atmospheric reentry environment. The oxidation properties of the samples were determined by measuring the weight gain per unit surface area and the thicknesses of the oxide scale. The oxide scale consists of a SiO2 outer layer, porous HfO2 layers, and an HfB2 layer depleted in SiC. A transition in HfO2 morphology from equixed to columnar and a decrease in SiO2 viscosity between 1800 and 1900 A degrees C accompanied a rapid increase in weight gain and scale thickness.