Mullite is considered a promising candidate for ceramic recuperators in turbo propelled engines, due to its low thermal conductivity, adequate thermal shock resistance, low cost, low density, thermodynamic stability, and reasonable strength at high temperatures. Unfortunately, the limited fracture toughness of mullite (~1.8-2.8 MPa m1/2) is considered too low. A reliable way of improving fracture toughness in a range of materials has been to tailor the microstructure to contain elongated grains capable of bridging cracks. In this thesis the tailoring of mullite microstructures using a range of processing methods is reported: reactive sintering of mixtures of alumina and silica, sol-gel synthesis of mullite and the use of sol-gel derived additives to enhance the sintering of commercial mullite powders. The differences in morphologies produced as well as the influence on indentation fracture toughness is described. The addition of ceria stabilised zirconia has been shown to improve the room temperature toughness of mullite to 4.7 MPa m1/2. In this work the toughening mechanisms of this and two other mullite zirconia composites (monoclinic zirconia and yttria stabilised zirconia) are investigated, as well as the effect of temperature on the toughness of the composites. It is unknown what effect the addition of the zirconia materials to mullite has on the other properties of the material. In this work the effect on the creep resistance is investigated, with mechanistic observations presented.