The corrosion products arising from the exposure of a Nicalon silicon carbide fibre-reinforced lithium aluminosilicate glass-ceramic matrix composite to molten sodium sulphate at 900 degrees C for 100 h in both oxygen and argon atmospheres were studied by X-ray diffraction (XRD) and scanning and transmission electron microscopy (SEM and TEM respectively). The microstructure of the as-received composite plates was found to be similar to that reported by other workers. The matrix consisted of grains of close to stoichiometric mullite and beta-spodumene and a high silica glass with 20-50 nm wide fibre-matrix interfaces comprising a layer of turbostratic carbon and amorphous silica. The effects of hot sodium sulphate corrosion were found to be very similar in both argon and oxygen but proceeded at a much greater rate in the latter case where it had progressed 100 mu m into the composite and consumed many fibres. XRD studies indicated that mullite had virtually disappeared in the corroded region and this was confirmed by SEM. TEM studies of thin sections cut from near the end of the corroded zone also showed that the matrix had become a very fine mixture of glass and beta-spodumene grains and that the fibre-matrix interface region had grown to ca. 600-800 nm wide. The microstructure of this corroded interface comprised several alternating layers of turbostratic carbon, mixed carbon and amorphous silica and pure carbon, each with widths varying between ca. 100 and 200 nm. This layered structure apparently developed as a result of oxidation of the silicon carbide fibre in the presence of a gradient of oxygen partial pressure that decreased from the matrix across the interface to the fibre.