Unidirectional silicon carbide fibre-reinforced calcium aluminosilicate (GAS) has been subjected to a variety of thermal regimes. Microscopy has been used to assess the degree of matrix damage. Thermal shock induced matrix cracking was first seen on the end faces of the composite, perpendicular to the fibre direction at a temperature differential of 400 degrees C. At more severe thermal shocks the next damage was observed on faces parallel to the fibre direction in the form of cracking in the matrix perpendicular to the fibre direction. Matrix cracking damage increased, initially, with increasing severity of thermal shock, but then became less extensive at the highest temperature differentials (800 degrees C) used. Thermal shock-induced crack densities were correlated with literature data for cracking under quasi-static loading using a simple thermal shock analysis incorporating a Stress reduction factor. The suitability of applying a modified Aveston, Cooper and Kelly (ACK) model [1] to predict critical temperature differentials for matrix cracking onset in the unidirectional composite has also been tested. The method was found to be valid for the unidirectional material providing that some key parameters were determined independently.