The thermomechanical relationship between mullite and a lithium aluminosilicate (LAS) glass‐ceramic has been studied by a technique of bimaterial dilatometry. The viscoelastic behavior of LAS was found to result in a temperatureTrvslbelow the glass‐transformation temperatureTgat which the interfacial stresses between the materials undergo a reversal in sign. The effect ofTrvslon the high‐temperature mechanical properties of mullite or SiC fiber‐reinforced glass‐ceramic composites is discussed. It is suggested that fiber clamping by the matrix at temperatures betweenTrvslandTgcontributes to the brittle, high‐temperature behavior of such