The relationships between mechanical properties and microstructure of alumina-mullite-zircon refractories were investigated by varying zircon content. Various sintering temperatures were used for the slip-cast refractories and mechanical properties determined as a function of zircon content. Strength and modulus values increased significantly (by a factor of similar to3) as a result of an increase in both sintering temperature and zircon content. The incorporation of fine zircon particles gave a significant improvement in densification through a reduction in porosity by faster sintering and by filling of interparticle voids due to the formation of a glassy phase. Fracture surface energy and critical crack size did not change significantly with increasing sintering temperature. However, the addition of increasing amount of zircon and the rise in the sintering temperature increased fracture toughness markedly, by a factor of similar to3 and similar to35%, respectively. Thermal shock parameter R''' showed a >10% improvement with increasing sintering temperature. Thermal shock tests confirmed that the refractory material, containing 20% zircon, sintered at 1650 degreesC showed the highest resistance to further thermal shock damage.