High-temperature mechanical behaviour of Nicalon/CAS-II composite has been investigated. Oxidation of the exposed interfaces along matrix cracks at 1000 degrees C lowered the longitudinal unidirectional strength to the stress level at which matrix cracking began to occur. The strength of cross-plied composites was also severely reduced in 800 degrees C air. Transverse plies cracked prior to 0 degrees ply matrix cracking. However, embrittlement did not occur until the matrix in the 0 degrees plies cracked. It was established that oxidation does not take part in crack growth parallel to the fibres, except adjacent to exposed edges. Neither does oxygen enter 90 degrees ply cracks in cross-plied composites in sufficient quantity to produce oxidation embrittlement, at least up to the 0 degrees matrix cracking strain.