Thermal shock parameters (R, R'" and R"") have been calculated using measured strength and modulus values for model magnesia-spinel composite materials. The R'" and R"" parameters vary with both spinel content (0-30%) and spinel particle size (3-22 mu-m). In general, the larger the spinel content, the higher the values of R'" and R"". It is predicted that the coarsest (22 mu-m) spinel provides a significant improvement in resistance to thermal shock damage through maximised difficulty of crack propagation, with a maximum at approximately 20% addition. These predictions were also matched by thermal shock testing. After quenching from 1000 degreesC, the 20% 22 mu-m spinel composite had a retained strength approximately 4.5 times higher than that for similarly quenched pure magnesia. (C) 2002 Elsevier Science Ltd. All rights reserved.