Carbon-carbon composites have been prepared with and without addition of 15 wt% of the glass-forming oxidation inhibitors boron and zirconium diboride. Samples of the composites carbonised at 850Β°C were further heat treated over a range of heat-treatment temperatures to 2650Β°C. Mechanical properties of the heat-treated composites were measured by 3-point bend testing and their oxidation behaviour at 850Β°C in air determined by TGA. The mechanical properties of composites without inhibitor additives were unaffected by heat treatment to 2250Β°C. However, at 2650Β°C strength and modulus were reduced. Composites containing inhibitors exhibited reduced mechanical properties on heat treatment above their carbonisation temperature, with the most marked reduction above 2300Β°C. Scanning electron microscopy showed that at temperatures above 2320Β°C with boron and 2330Β°C with ZrB2 the carbonaceous-composite components reacted with the inhibitors, probably forming carbides. The oxidation resistance of composites without inhibitors and those containing ZrBl increased with increasing heattreatment temperature to a maximum at 2250Β°C before decreasing at 2650Β°C. Boron appeared the most effective oxidation inhibitor. The oxidation behaviour of the inhibitor-loaded composites appeared unaffected by the reaction of the carbonaceous-composite components and the inhibitors at temperatures above 2320Β°C.