Ceramic-matrix composites offer considerable advantages over single-phase materials for high-temperature applications, in view of their enhanced fracture toughness, non-catastrophic failure mode and increased thermal shock resistance. From the perspective of thermal insulation, temperature control and energy conservation, information on the variables that control the effective thermal conductivity of ceramic-matrix composites also is critical for purposes of materials selection and performance prediction of high-temperature structures and components. Earlier theoretical studies have shown that the effective thermal conductivity of composites depends on the values for the thermal conductivity, the volume fractions and the distribution of the individual components within the composite [1-6]. More recently it was shown, both analytically and experimentally, that an interracial thermal barrier can also play a significant role in the effective thermal conductivity of composite materials [7-12]. In general, the aforementioned theories show that, regardless of the phase distribution of the individual components, an increase or decrease in the thermal conductivity of any of the components will result in a corresponding change in the effective thermal conductivity of the composite, regardless of the direction in which it is measured. Recently it was observed that following heating of a biaxial weave SiC fibre-reinforced chemical vapour deposited (CVD) SiC matrix composite to 1500 °C, the effective thermal diffusivity within the plane of the fibre showed an increase, whereas the thermal diffusivity transverse to the fibre plane showed a decrease, in apparent contradiction to theory. It was also observed that, following heating to 1500 °C, the thermal diffusivity transverse to the fibre plane exhibited different values in vacuum than in helium or nitrogen at atmospheric pressure. It is the purpose of this letter to report these observations and to offer an explanation. The composite samples consisted of a 0/90 ° bidirectional satin weave of very fine crystalline Nicalon SiC fibres. The fibre weave was densified by chemical vapour infiltration (CVI) of SiC from mixtures of methyltrichlorosilane and hydrogen at a temperature of about 1000 °C. Following the CVI