A unidirectional composite and a series of bidirectionally reinforced composites were fabricated using carbon fibre reinforcement in a silicon carbide matrix, which was produced by the pyrolysis of a polymer precursor. The thermal expansion over the temperature range 20-1000 degrees C has been measured and the thermal diffusivity measured over the temperature range 200-1200 degrees C. Thermal diffusivity data was converted to conductivity data using measured density and literature specific heat data. Metallographic examination has been carried out on the composites and the results are discussed in terms of the observed microstructural features.