Long, square cross-section samples of a unidirectional and a cross-ply [0/90](3s) silicon carbide (Nicalon) fibre calcium aluminosilicate glass-ceramic matrix composite have been subjected to a range of thermal treatments. They were held at temperatures up to 800 degrees C above room temperature for 1, 6 or 24 h then slowly cooled or quenched into water. The thermal cycle was repeated up to six times for a small number of samples. The effects of these thermal regimes on Young＇s modulus, onset of matrix cracking (as assessed by onset of non-linearity in the load-displacement curve) and flexure strength have been monitored using three-point flexure testing. In very broad terms, where clear trends emerged, the intermediate temperature differentials, i.e. 400-650 degrees C, have been found to have the most detrimental effects on properties, and this has been linked to expected changes in the carboneous interphase and its subsequent replacement by silica.